Screw (simple machine)
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Look up screw in
Wiktionary, the free dictionary.
A screw is one of the six simple machines. All screws are helical inclined planes. A screw can convert a rotational force (torque) to a linear force and vice versa. The ratio of threading determines the mechanical advantage of the machine. More threading increases the mechanical advantage. A rough comparison of mechanical advantage can be done by taking the circumference of the shaft of the screw and divide by the distance between the threads.
A screw is a shaft with a helical groove or thread formed on its surface and provision at one end to turn the screw. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force. It can also be defined as an inclined plane wrapped around a shaft.
Screws come in a variety of shapes and sizes for different purposes.
Thread as found on a screw.
[edit] Examples
* Lead screws and ball screws are specialized screws for translating rotational to linear motion.
* Automated garage doors, where a motor drives a long finely threaded shaft at relatively high speed and lifts the heavy door at a slower rate.
* Archimedes' screw and worm gears are examples of this machine.
Thursday, March 12, 2009
♣weDge♣
Wedge (mechanical device)
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Wedge
Wood splitting wedge
Classification Hand tool
Used with Sledgehammer
Related Chisel
Splitting maul
Axe
A wedge is a triangular shaped tool, a compound and portable inclined plane, and one of the six classical simple machines. It can be used to separate two objects or portions of an object, lift an object, or hold an object in place. It functions by converting a force applied to its blunt end into forces perpendicular to its length. The mechanical advantage of a wedge is given by the ratio of its length to its width.[1] Although a short wedge with a wide angle may do a job faster, it requires more force than a long wedge with a narrow angle.
Contents
[hide]
* 1 History
* 2 Examples for lifting and separating
* 3 Examples for holding fast
* 4 Mechanical advantage
* 5 References
* 6 See also
* 7 External links
[edit] History
The origin of the wedge is unknown likely because it has been in use for over 9000 years. In ancient Egyptian quarries, bronze wedges were used to break away blocks of stone used in construction. Wooden wedges, that swelled after being saturated with water, were also used. Some indigenous peoples of the Americas used antler wedges for splitting and working wood to make canoes, dwellings and other objects.
[edit] Examples for lifting and separating
Wedges can be used to lift heavy objects, separating them from the surface they rest on. They can also be used to separate objects, such as blocks of cut stone. Splitting mauls and splitting wedges are used to split wood along the grain. A narrow wedge with a relatively long taper used to finely adjust the distance between objects is called a shim, and is commonly used in carpentry.
The tips of forks and nails are also wedges, as they split and separate the material into which they are pushed or driven; the shafts may then hold fast due to friction.
[edit] Examples for holding fast
An insect nest is wedged in between two stones to hold it in place.
Wedges can also be used to hold objects in place, such as engine parts (poppet valves), bicycle parts (stems and eccentric bottom brackets), and doors.
A wedge-type door stop (door wedge) functions largely because of the friction generated between the bottom of the door and the wedge, and the wedge and the floor (or other surface).
[edit] Mechanical advantage
Cross-section of a wedge with its length oriented vertically. A downward force produces horizontal forces extending outward.
The mechanical advantage of a wedge can be calculated by dividing its length by its width as follows:
MA={L \over W}
The more "acute" (narrow) the angle of a wedge, the greater the ratio of its length to its width, and thus the more mechanical advantage it will yield.
However, in an elastic material such as wood, friction may bind a narrow wedge more easily than a wide one. This is why the head of a splitting maul has a much wider angle than that of an axe.
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Wedge
Wood splitting wedge
Classification Hand tool
Used with Sledgehammer
Related Chisel
Splitting maul
Axe
A wedge is a triangular shaped tool, a compound and portable inclined plane, and one of the six classical simple machines. It can be used to separate two objects or portions of an object, lift an object, or hold an object in place. It functions by converting a force applied to its blunt end into forces perpendicular to its length. The mechanical advantage of a wedge is given by the ratio of its length to its width.[1] Although a short wedge with a wide angle may do a job faster, it requires more force than a long wedge with a narrow angle.
Contents
[hide]
* 1 History
* 2 Examples for lifting and separating
* 3 Examples for holding fast
* 4 Mechanical advantage
* 5 References
* 6 See also
* 7 External links
[edit] History
The origin of the wedge is unknown likely because it has been in use for over 9000 years. In ancient Egyptian quarries, bronze wedges were used to break away blocks of stone used in construction. Wooden wedges, that swelled after being saturated with water, were also used. Some indigenous peoples of the Americas used antler wedges for splitting and working wood to make canoes, dwellings and other objects.
[edit] Examples for lifting and separating
Wedges can be used to lift heavy objects, separating them from the surface they rest on. They can also be used to separate objects, such as blocks of cut stone. Splitting mauls and splitting wedges are used to split wood along the grain. A narrow wedge with a relatively long taper used to finely adjust the distance between objects is called a shim, and is commonly used in carpentry.
The tips of forks and nails are also wedges, as they split and separate the material into which they are pushed or driven; the shafts may then hold fast due to friction.
[edit] Examples for holding fast
An insect nest is wedged in between two stones to hold it in place.
Wedges can also be used to hold objects in place, such as engine parts (poppet valves), bicycle parts (stems and eccentric bottom brackets), and doors.
A wedge-type door stop (door wedge) functions largely because of the friction generated between the bottom of the door and the wedge, and the wedge and the floor (or other surface).
[edit] Mechanical advantage
Cross-section of a wedge with its length oriented vertically. A downward force produces horizontal forces extending outward.
The mechanical advantage of a wedge can be calculated by dividing its length by its width as follows:
MA={L \over W}
The more "acute" (narrow) the angle of a wedge, the greater the ratio of its length to its width, and thus the more mechanical advantage it will yield.
However, in an elastic material such as wood, friction may bind a narrow wedge more easily than a wide one. This is why the head of a splitting maul has a much wider angle than that of an axe.
fulcrum
Fulcrum
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Sister project Look up fulcrum in
Wiktionary, the free dictionary.
Fulcrum may refer to one of the following.
* Fulcrum, the pivot on which a lever moves
* Fulcrum Wheels, a bicycle wheel manufacturer, based in Italy
* Fulcrum (drumming), part of a percussionist's grip
* MiG-29 Fulcrum or Mikoyan MiG-29, a Soviet fighter aircraft
* Fulcrum (Anglican think tank), a Church of England think tank
* Fulcrum (newspaper), a student newspaper at the University of Ottawa
* Fulcrum (annual), a United States literary periodical, an annual of poetry and aesthetics
* Fulcrum Technologies, a former Canadian search engine, now part of Open Text Corporation
* Fulcrum (Chuck), the enemy spy organization on the TV series Chuck
This disambiguation page lists articles associated with the same title. If an internal link led you here, you may wish to change the link to point directly to the intended article.
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Sister project Look up fulcrum in
Wiktionary, the free dictionary.
Fulcrum may refer to one of the following.
* Fulcrum, the pivot on which a lever moves
* Fulcrum Wheels, a bicycle wheel manufacturer, based in Italy
* Fulcrum (drumming), part of a percussionist's grip
* MiG-29 Fulcrum or Mikoyan MiG-29, a Soviet fighter aircraft
* Fulcrum (Anglican think tank), a Church of England think tank
* Fulcrum (newspaper), a student newspaper at the University of Ottawa
* Fulcrum (annual), a United States literary periodical, an annual of poetry and aesthetics
* Fulcrum Technologies, a former Canadian search engine, now part of Open Text Corporation
* Fulcrum (Chuck), the enemy spy organization on the TV series Chuck
This disambiguation page lists articles associated with the same title. If an internal link led you here, you may wish to change the link to point directly to the intended article.
Complex Machines
By Sharon Fabian
1 Let's say you already know about the six simple machines: inclined plane, wedge, screw, lever, wheel and axle, and pulley. You've probably figured out that these six machines were invented ages and ages ago. So what has been happening since then? Did someone invent simple machine number 7, 8, 9, 10, and so on? How many simple machines do we have by now? Hundreds? Thousands?
2 Lets look at a bicycle for an example. A bicycle is a much newer machine than the simple lever that a cave man used to move a big old rock, but its not as new as, say, a laptop computer. Where does a bicycle fit into the world of machines? Well, a bicycle is not number 7, or 100, or even 1000. A bicycle is actually a combination of several of those six basic simple machines. A bicycle gear is actually a combination of simple machines all by itself. A gear is a wheel, but the teeth on the gear are little wedges. What other simple machines can you find on a bicycle?
3 Gears, along with other simple machines, make up many of the machines you use every day. Some examples are the lawn sprinkler, a watch, and the gearbox in a car.
By Sharon Fabian
1 Let's say you already know about the six simple machines: inclined plane, wedge, screw, lever, wheel and axle, and pulley. You've probably figured out that these six machines were invented ages and ages ago. So what has been happening since then? Did someone invent simple machine number 7, 8, 9, 10, and so on? How many simple machines do we have by now? Hundreds? Thousands?
2 Lets look at a bicycle for an example. A bicycle is a much newer machine than the simple lever that a cave man used to move a big old rock, but its not as new as, say, a laptop computer. Where does a bicycle fit into the world of machines? Well, a bicycle is not number 7, or 100, or even 1000. A bicycle is actually a combination of several of those six basic simple machines. A bicycle gear is actually a combination of simple machines all by itself. A gear is a wheel, but the teeth on the gear are little wedges. What other simple machines can you find on a bicycle?
3 Gears, along with other simple machines, make up many of the machines you use every day. Some examples are the lawn sprinkler, a watch, and the gearbox in a car.
♥☺☻resistance☻☺♥
Electrical resistance
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Electromagnetism
Electricity · Magnetism
[show]Electrostatics
Electric charge · Coulomb's law · Electric field · Electric flux · Gauss's law · Electric potential · Electrostatic induction · Electric dipole moment ·
[show]Magnetostatics
Ampère’s law · Electric current · Magnetic field · Magnetic flux · Biot–Savart law · Magnetic dipole moment · Gauss’s law for magnetism ·
[show]Electrodynamics
Free space · Lorentz force law · EMF · Electromagnetic induction · Faraday’s law · Displacement current · Maxwell’s equations · EM field · Electromagnetic radiation · Liénard-Wiechert Potential · Maxwell tensor · Eddy current ·
[show]Electrical Network
Electrical conduction · Electrical resistance · Capacitance · Inductance · Impedance · Resonant cavities · Waveguides ·
[show]Covariant formulation
Electromagnetic tensor · EM Stress-energy tensor · Four-current · Four-potential ·
[show]Scientists
Ampère · Coulomb · Faraday · Heaviside · Henry · Hertz · Lorentz · Maxwell · Tesla · Weber ·
This box: view • talk • edit
A 750-kΩ resistor, as identified by its electronic color code. An ohmmeter could be used to verify this value.
The electrical resistance of an object is a measure of its opposition to the passage of a steady electrical current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material.
Discovered by Georg Ohm in the late 1820s[1], electrical resistance shares some conceptual parallels with the mechanical notion of friction. The SI unit of electrical resistance is the ohm, symbol Ω. Resistance's reciprocal quantity is electrical conductance measured in siemens, symbol S.
The resistance of a resistive object determines the amount of current through the object for a given potential difference across the object, in accordance with Ohm's law:
I = {V \over R}
where
R is the resistance of the object, measured in ohms, equivalent to J·s/C2
V is the potential difference across the object, measured in volts
I is the current through the object, measured in amperes
For a wide variety of materials and conditions, the electrical resistance does not depend on the amount of current through or the amount of voltage across the object, meaning that the resistance R is constant for the given temperature. Therefore, the resistance of an object can be defined as the ratio of voltage to current:
R = {V \over I}
In the case of nonlinear objects (not purely resistive, or not obeying Ohm's law), this ratio can change as current or voltage changes; the ratio taken at any particular point, the inverse slope of a chord to an I–V curve, is sometimes referred to as a "chordal resistance" or "static resistance".[2][3]
Contents
[hide]
* 1 Resistance of a conductor
o 1.1 DC resistance
o 1.2 AC resistance
* 2 Causes of resistance
o 2.1 In metals
o 2.2 In semiconductors and insulators
o 2.3 In ionic liquids/electrolytes
o 2.4 Resistivity of various materials
o 2.5 Band theory simplified
* 3 Differential resistance
* 4 Temperature-dependence
* 5 Measuring resistance
* 6 See also
* 7 References
* 8 External links
[edit] Resistance of a conductor
[edit] DC resistance
The resistance R of a conductor of uniform cross section can be computed as
R = {\ell \cdot \rho \over A} \,
where
ℓ is the length of the conductor, measured in meters
A is the cross-sectional area, measured in square meters
ρ (Greek: rho) is the electrical resistivity (also called specific electrical resistance) of the material, measured in Ohm · meter. Resistivity is a measure of the material's ability to oppose electric current.
For practical reasons, any connections to a real conductor will almost certainly mean the current density is not totally uniform. However, this formula still provides a good approximation for long thin conductors such as wires.
[edit] AC resistance
If a wire conducts high-frequency alternating current then the effective cross sectional area of the wire is reduced because of the skin effect. If several conductors are together, then due to proximity effect, the effective resistance of each is higher than if that conductor were alone.
[edit] Causes of resistance
[edit] In metals
A metal consists of a lattice of atoms, each with a shell of electrons. This can also be known as a positive ionic lattice. The outer electrons are free to dissociate from their parent atoms and travel through the lattice, creating a 'sea' of electrons, making the metal a conductor. When an electrical potential difference (a voltage) is applied across the metal, the electrons drift from one end of the conductor to the other under the influence of the electric field.
Near room temperatures, the thermal motion of ions is the primary source of scattering of electrons (due to destructive interference of free electron waves on non-correlating potentials of ions), and is thus the prime cause of metal resistance. Imperfections of lattice also contribute into resistance, although their contribution in pure metals is negligible.
The larger the cross-sectional area of the conductor, the more electrons are available to carry the current, so the lower the resistance. The longer the conductor, the more scattering events occur in each electron's path through the material, so the higher the resistance. Different materials also affect the resistance.[1]
[edit] In semiconductors and insulators
In metals, the Fermi level lies in the conduction band (see Band Theory, below) giving rise to free conduction electrons. However, in semiconductors the position of the Fermi level is within the band gap, approximately half-way between the conduction band minimum and valence band maximum for intrinsic (undoped) semiconductors. This means that at 0 Kelvin, there are no free conduction electrons and the resistance is infinite. However, the resistance will continue to decrease as the charge carrier density in the conduction band increases. In extrinsic (doped) semiconductors, dopant atoms increase the majority charge carrier concentration by donating electrons to the conduction band or accepting holes in the valence band. For both types of donor or acceptor atoms, increasing the dopant density leads to a reduction in the resistance. Highly doped semiconductors hence behave metallic. At very high temperatures, the contribution of thermally generated carriers will dominate over the contribution from dopant atoms and the resistance will decrease exponentially with temperature.
[edit] In ionic liquids/electrolytes
In electrolytes, electrical conduction happens not by band electrons or holes, but by full atomic species (ions) traveling, each carrying an electrical charge. The resistivity of ionic liquids varies tremendously by the concentration - while distilled water is almost an insulator, salt water is a very efficient electrical conductor. In biological membranes, currents are carried by ionic salts. Small holes in the membranes, called ion channels, are selective to specific ions and determine the membrane resistance.
[edit] Resistivity of various materials
Main article: electrical resistivities of the elements (data page)
Material Resistivity, ρ
ohm-meter
Metals 10 - 8
Semiconductors variable
Electrolytes variable
Insulators 1016
Superconductors 0 (exactly)
[edit] Band theory simplified
Electron energy levels in an insulator.
Quantum mechanics states that the energy of an electron in an atom cannot be any arbitrary value. Rather, there are fixed energy levels which the electrons can occupy, and values in between these levels are impossible. The energy levels are grouped into two bands: the valence band and the conduction band (the latter is generally above the former). Electrons in the conduction band may move freely throughout the substance in the presence of an electrical field.
In insulators and semiconductors, the atoms in the substance influence each other so that between the valence band and the conduction band there exists a forbidden band of energy levels, which the electrons cannot occupy. In order for a current to flow, a relatively large amount of energy must be furnished to an electron for it to leap across this forbidden gap and into the conduction band. Thus, even large voltages can yield relatively small currents.
[edit] Differential resistance
When resistance may depend on voltage and current, differential resistance, incremental resistance or slope resistance is defined as the slope of the V-I graph at a particular point, thus:
R = \frac {\mathrm{d}V} {\mathrm{d}I} \,
This quantity is sometimes called simply resistance, although the two definitions are equivalent only for an ohmic component such as an ideal resistor. For example, a diode is a circuit element for which the resistance depends on the applied voltage or current.
If the V-I graph is not monotonic (i.e. it has a peak or a trough), the differential resistance will be negative for some values of voltage and current. This property is often known as negative resistance, although it is more correctly called negative differential resistance, since the absolute resistance V/I is still positive. Example of such an element is a tunnel diode.
[edit] Temperature-dependence
Near room temperature, the electric resistance of a typical metal increases linearly with rising temperature, while the electrical resistance of a typical semiconductor decreases with rising temperature. The amount of that change in resistance can be calculated using the temperature coefficient of resistivity of the material.
At lower temperatures (less than the Debye temperature), the resistance of a metal decreases as T5 due to the electrons scattering off of phonons. At even lower temperatures, the dominant scattering mechanism for electrons is other electrons, and the resistance decreases as T2. At some point, the impurities in the metal will dominate the behavior of the electrical resistance which causes it to saturate to a constant value. Matthiessen's Rule (first formulated by Augustus Matthiessen in the 1860s; the equation below gives its modern form) [4][5] says that all of these different behaviors can be summed up to get the total resistance as a function of temperature,
R = R_\text{imp} + a T^2 + b T^5 + cT \,
where Rimp is the temperature independent electrical resistivity due to impurities, and a, b, and c are coefficients which depend upon the metal's properties. This rule can be seen as the motivation to Heike Kamerlingh Onnes's experiments that lead in 1911 to discovery of superconductivity. For details see History of superconductivity.
The electric resistance of a typical intrinsic (non doped) semiconductor decreases exponentially with the temperature:
R= R_0 e^{-aT}\,
Extrinsic (doped) semiconductors have a far more complicated temperature profile. As temperature increases starting from absolute zero they first decrease steeply in resistance as the carriers leave the donors or acceptors. After most of the donors or acceptors have lost their carriers the resistance starts to increase again slightly due to the reducing mobility of carriers (much as in a metal). At higher temperatures it will behave like intrinsic semiconductors as the carriers from the donors/acceptors become insignificant compared to the thermally generated carriers.
The electric resistance of electrolytes and insulators is highly nonlinear, and case by case dependent, therefore no generalized equations are given.
[edit] Measuring resistance
An instrument for measuring resistance is called an ohmmeter. Simple ohmmeters cannot measure low resistances accurately because the resistance of their measuring leads causes a voltage drop that interferes with the measurement, so more accurate devices use four-terminal sensing.
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Electromagnetism
Electricity · Magnetism
[show]Electrostatics
Electric charge · Coulomb's law · Electric field · Electric flux · Gauss's law · Electric potential · Electrostatic induction · Electric dipole moment ·
[show]Magnetostatics
Ampère’s law · Electric current · Magnetic field · Magnetic flux · Biot–Savart law · Magnetic dipole moment · Gauss’s law for magnetism ·
[show]Electrodynamics
Free space · Lorentz force law · EMF · Electromagnetic induction · Faraday’s law · Displacement current · Maxwell’s equations · EM field · Electromagnetic radiation · Liénard-Wiechert Potential · Maxwell tensor · Eddy current ·
[show]Electrical Network
Electrical conduction · Electrical resistance · Capacitance · Inductance · Impedance · Resonant cavities · Waveguides ·
[show]Covariant formulation
Electromagnetic tensor · EM Stress-energy tensor · Four-current · Four-potential ·
[show]Scientists
Ampère · Coulomb · Faraday · Heaviside · Henry · Hertz · Lorentz · Maxwell · Tesla · Weber ·
This box: view • talk • edit
A 750-kΩ resistor, as identified by its electronic color code. An ohmmeter could be used to verify this value.
The electrical resistance of an object is a measure of its opposition to the passage of a steady electrical current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material.
Discovered by Georg Ohm in the late 1820s[1], electrical resistance shares some conceptual parallels with the mechanical notion of friction. The SI unit of electrical resistance is the ohm, symbol Ω. Resistance's reciprocal quantity is electrical conductance measured in siemens, symbol S.
The resistance of a resistive object determines the amount of current through the object for a given potential difference across the object, in accordance with Ohm's law:
I = {V \over R}
where
R is the resistance of the object, measured in ohms, equivalent to J·s/C2
V is the potential difference across the object, measured in volts
I is the current through the object, measured in amperes
For a wide variety of materials and conditions, the electrical resistance does not depend on the amount of current through or the amount of voltage across the object, meaning that the resistance R is constant for the given temperature. Therefore, the resistance of an object can be defined as the ratio of voltage to current:
R = {V \over I}
In the case of nonlinear objects (not purely resistive, or not obeying Ohm's law), this ratio can change as current or voltage changes; the ratio taken at any particular point, the inverse slope of a chord to an I–V curve, is sometimes referred to as a "chordal resistance" or "static resistance".[2][3]
Contents
[hide]
* 1 Resistance of a conductor
o 1.1 DC resistance
o 1.2 AC resistance
* 2 Causes of resistance
o 2.1 In metals
o 2.2 In semiconductors and insulators
o 2.3 In ionic liquids/electrolytes
o 2.4 Resistivity of various materials
o 2.5 Band theory simplified
* 3 Differential resistance
* 4 Temperature-dependence
* 5 Measuring resistance
* 6 See also
* 7 References
* 8 External links
[edit] Resistance of a conductor
[edit] DC resistance
The resistance R of a conductor of uniform cross section can be computed as
R = {\ell \cdot \rho \over A} \,
where
ℓ is the length of the conductor, measured in meters
A is the cross-sectional area, measured in square meters
ρ (Greek: rho) is the electrical resistivity (also called specific electrical resistance) of the material, measured in Ohm · meter. Resistivity is a measure of the material's ability to oppose electric current.
For practical reasons, any connections to a real conductor will almost certainly mean the current density is not totally uniform. However, this formula still provides a good approximation for long thin conductors such as wires.
[edit] AC resistance
If a wire conducts high-frequency alternating current then the effective cross sectional area of the wire is reduced because of the skin effect. If several conductors are together, then due to proximity effect, the effective resistance of each is higher than if that conductor were alone.
[edit] Causes of resistance
[edit] In metals
A metal consists of a lattice of atoms, each with a shell of electrons. This can also be known as a positive ionic lattice. The outer electrons are free to dissociate from their parent atoms and travel through the lattice, creating a 'sea' of electrons, making the metal a conductor. When an electrical potential difference (a voltage) is applied across the metal, the electrons drift from one end of the conductor to the other under the influence of the electric field.
Near room temperatures, the thermal motion of ions is the primary source of scattering of electrons (due to destructive interference of free electron waves on non-correlating potentials of ions), and is thus the prime cause of metal resistance. Imperfections of lattice also contribute into resistance, although their contribution in pure metals is negligible.
The larger the cross-sectional area of the conductor, the more electrons are available to carry the current, so the lower the resistance. The longer the conductor, the more scattering events occur in each electron's path through the material, so the higher the resistance. Different materials also affect the resistance.[1]
[edit] In semiconductors and insulators
In metals, the Fermi level lies in the conduction band (see Band Theory, below) giving rise to free conduction electrons. However, in semiconductors the position of the Fermi level is within the band gap, approximately half-way between the conduction band minimum and valence band maximum for intrinsic (undoped) semiconductors. This means that at 0 Kelvin, there are no free conduction electrons and the resistance is infinite. However, the resistance will continue to decrease as the charge carrier density in the conduction band increases. In extrinsic (doped) semiconductors, dopant atoms increase the majority charge carrier concentration by donating electrons to the conduction band or accepting holes in the valence band. For both types of donor or acceptor atoms, increasing the dopant density leads to a reduction in the resistance. Highly doped semiconductors hence behave metallic. At very high temperatures, the contribution of thermally generated carriers will dominate over the contribution from dopant atoms and the resistance will decrease exponentially with temperature.
[edit] In ionic liquids/electrolytes
In electrolytes, electrical conduction happens not by band electrons or holes, but by full atomic species (ions) traveling, each carrying an electrical charge. The resistivity of ionic liquids varies tremendously by the concentration - while distilled water is almost an insulator, salt water is a very efficient electrical conductor. In biological membranes, currents are carried by ionic salts. Small holes in the membranes, called ion channels, are selective to specific ions and determine the membrane resistance.
[edit] Resistivity of various materials
Main article: electrical resistivities of the elements (data page)
Material Resistivity, ρ
ohm-meter
Metals 10 - 8
Semiconductors variable
Electrolytes variable
Insulators 1016
Superconductors 0 (exactly)
[edit] Band theory simplified
Electron energy levels in an insulator.
Quantum mechanics states that the energy of an electron in an atom cannot be any arbitrary value. Rather, there are fixed energy levels which the electrons can occupy, and values in between these levels are impossible. The energy levels are grouped into two bands: the valence band and the conduction band (the latter is generally above the former). Electrons in the conduction band may move freely throughout the substance in the presence of an electrical field.
In insulators and semiconductors, the atoms in the substance influence each other so that between the valence band and the conduction band there exists a forbidden band of energy levels, which the electrons cannot occupy. In order for a current to flow, a relatively large amount of energy must be furnished to an electron for it to leap across this forbidden gap and into the conduction band. Thus, even large voltages can yield relatively small currents.
[edit] Differential resistance
When resistance may depend on voltage and current, differential resistance, incremental resistance or slope resistance is defined as the slope of the V-I graph at a particular point, thus:
R = \frac {\mathrm{d}V} {\mathrm{d}I} \,
This quantity is sometimes called simply resistance, although the two definitions are equivalent only for an ohmic component such as an ideal resistor. For example, a diode is a circuit element for which the resistance depends on the applied voltage or current.
If the V-I graph is not monotonic (i.e. it has a peak or a trough), the differential resistance will be negative for some values of voltage and current. This property is often known as negative resistance, although it is more correctly called negative differential resistance, since the absolute resistance V/I is still positive. Example of such an element is a tunnel diode.
[edit] Temperature-dependence
Near room temperature, the electric resistance of a typical metal increases linearly with rising temperature, while the electrical resistance of a typical semiconductor decreases with rising temperature. The amount of that change in resistance can be calculated using the temperature coefficient of resistivity of the material.
At lower temperatures (less than the Debye temperature), the resistance of a metal decreases as T5 due to the electrons scattering off of phonons. At even lower temperatures, the dominant scattering mechanism for electrons is other electrons, and the resistance decreases as T2. At some point, the impurities in the metal will dominate the behavior of the electrical resistance which causes it to saturate to a constant value. Matthiessen's Rule (first formulated by Augustus Matthiessen in the 1860s; the equation below gives its modern form) [4][5] says that all of these different behaviors can be summed up to get the total resistance as a function of temperature,
R = R_\text{imp} + a T^2 + b T^5 + cT \,
where Rimp is the temperature independent electrical resistivity due to impurities, and a, b, and c are coefficients which depend upon the metal's properties. This rule can be seen as the motivation to Heike Kamerlingh Onnes's experiments that lead in 1911 to discovery of superconductivity. For details see History of superconductivity.
The electric resistance of a typical intrinsic (non doped) semiconductor decreases exponentially with the temperature:
R= R_0 e^{-aT}\,
Extrinsic (doped) semiconductors have a far more complicated temperature profile. As temperature increases starting from absolute zero they first decrease steeply in resistance as the carriers leave the donors or acceptors. After most of the donors or acceptors have lost their carriers the resistance starts to increase again slightly due to the reducing mobility of carriers (much as in a metal). At higher temperatures it will behave like intrinsic semiconductors as the carriers from the donors/acceptors become insignificant compared to the thermally generated carriers.
The electric resistance of electrolytes and insulators is highly nonlinear, and case by case dependent, therefore no generalized equations are given.
[edit] Measuring resistance
An instrument for measuring resistance is called an ohmmeter. Simple ohmmeters cannot measure low resistances accurately because the resistance of their measuring leads causes a voltage drop that interferes with the measurement, so more accurate devices use four-terminal sensing.
Mechanical advantage
From Wikipedia, the free encyclopedia
Jump to: navigation, search
In physics and engineering, mechanical advantage (MA) is the factor by which a mechanism multiplies the force or torque put into it. Generally, the mechanical advantage is calculated as follows:
MA = \frac{\text{distance over which effort is applied}}{\text{distance over which the load is moved}}
or more simply:
MA = \frac{\text{output force}}{\text{input force}}
The first equation shows that the force exerted IN to the machine multiplied by the distance moved IN will always be equal to the force exerted OUT of the machine multiplied by the distance moved OUT. For example, using a block and tackle with 6 ropes, and a 600 pound load, the operator would be required to pull the rope 6 feet, and exert 100 pounds of force to lift the 600 pound load 1 foot.
The second equation is a simplified formula based just on the forces in and out. Using the example above, 100 pounds of force IN results in 600 pounds of force OUT, an MA of 6. Both of these equations calculate only the ideal mechanical advantage (IMA) and ignore any losses due to friction.
The actual mechanical advantage (AMA) includes those frictional losses. The difference between the two is the mechanical efficiency of the system.
Contents
[hide]
* 1 Simple machines
o 1.1 Pulleys
o 1.2 Screws
* 2 Types
o 2.1 Ideal mechanical advantage
o 2.2 Actual mechanical advantage
* 3 See also
* 4 References
o 4.1 Notes
o 4.2 Bibliography
* 5 External links
[edit] Simple machines
Beam balanced around a fulcrum
The following simple machines exhibit a mechanical advantage:
* The beam shown is in static equilibrium around the fulcrum. This is due to the moment created by vector force "A" counterclockwise (moment A*a) being in equilibrium with the moment created by vector force "B" clockwise (moment B*b). The relatively low vector force "B" is translated in a relatively high vector force "A". The force is thus increased in the ratio of the forces A : B, which is equal to the ratio of the distances to the fulcrum b : a. This ratio is called the mechanical advantage. This idealised situation does not take into account friction. For more explanation, see also lever.
* Wheel and axle notion (e.g. screwdrivers, doorknobs): A wheel is essentially a lever with one arm the distance between the axle and the outer point of the wheel, and the other the radius of the axle. Typically this is a fairly large difference, leading to a proportionately large mechanical advantage. This allows even simple wheels with wooden axles running in wooden blocks to still turn freely, because their friction is overwhelmed by the rotational force of the wheel multiplied by the mechanical advantage.
* Pulley: Pulleys change the direction of a tension force on a flexible material, e.g. a rope or cable. In addition, pulleys can be "added together" to create mechanical advantage, by having the flexible material looped over several pulleys in turn. Adding more loops and pulleys increases the mechanical advantage.
* Screw: A screw is essentially an inclined plane wrapped around a cylinder. The run over the rise of this inclined plane is the mechanical advantage of a screw.[1]
[edit] Pulleys
An example of a rope and pulley system illustrating mechanical advantage.
Consider lifting a weight with rope and pulleys. A rope looped through a pulley attached to a fixed spot, e.g. a barn roof rafter, and attached to the weight is called a single fixed pulley. It has an MA = 1 (assuming frictionless bearings in the pulley), meaning no mechanical advantage (or disadvantage) however advantageous the change in direction may be.
A single movable pulley has an MA of 2 (assuming frictionless bearings in the pulley). Consider a pulley attached to a weight being lifted. A rope passes around it, with one end attached to a fixed point above, e.g. a barn roof rafter, and a pulling force is applied upward to the other end with the two lengths parallel. In this situation the distance the lifter must pull the rope becomes twice the distance the weight travels, allowing the force applied to be halved. Note: if an additional pulley is used to change the direction of the rope, e.g. the person doing the work wants to stand on the ground instead of on a rafter, the mechanical advantage is not increased.
By looping more ropes around more pulleys we can continue to increase the mechanical advantage. For example if we have two pulleys attached to the rafter, two pulleys attached to the weight, one end attached to the rafter, and someone standing on the rafter pulling the rope, we have a mechanical advantage of four. Again note: if we add another pulley so that someone may stand on the ground and pull down, we still have a mechanical advantage of four.
Here are examples where the fixed point is not obvious:
* A velcro strap on a shoe passes through a slot and folds over on itself. The slot is a movable pulley and the MA = 2.
* Two ropes laid down a ramp attached to a raised platform. A barrel is rolled onto the ropes and the ropes are passed over the barrel and handed to two workers at the top of the ramp. The workers pull the ropes together to get the barrel to the top. The barrel is a movable pulley and the MA = 2. If there is enough friction where the rope is pinched between the barrel and the ramp, the pinch point becomes the attachment point. This is considered a fixed attachment point because the rope above the barrel does not move relative to the ramp. Alternatively the ends of the rope can be attached to the platform.
* Block and tackle: MA = 3
* Inclined plane: MA = length of slope ÷ height of slope
[edit] Screws
The theoretical mechanical advantage for a screw can be calculated using the following equation:[2]
MA = \frac{\pi d_m}{l}
where
dm = the mean diameter of the screw thread
l = the lead of the screw thread
Note that the actual mechanical advantage of a screw system is greater, as a screwdriver or other screw driving system has a mechanical advantage as well.
[edit] Types
There are two types of mechanical advantage:
1. Ideal mechanical advantage (IMA)
2. Actual mechanical advantage (AMA)
[edit] Ideal mechanical advantage
The ideal mechanical advantage (IMA), or theoretical mechanical advantage, is the mechanical advantage of an ideal machine. It is usually calculated using physics principles because there is no ideal machine.
The IMA of a machine can be found with the following formula:
IMA = \frac {D_E} {D_R}
where
DE equals the effort distance (the distance from the fulcrum to where the effort is applied)
DR equals the resistance distance (the distance from the fulcrum to where the resistance is applied)
[edit] Actual mechanical advantage
The actual mechanical advantage (AMA) is the mechanical advantage of a real machine. Actual mechanical advantage takes into consideration real world factors such as energy lost in friction.
The AMA of a machine is calculated with the following formula:
AMA = \frac {R} {E_\text{actual}}
where
R = resistance force
Eactual = actual effort force
From Wikipedia, the free encyclopedia
Jump to: navigation, search
In physics and engineering, mechanical advantage (MA) is the factor by which a mechanism multiplies the force or torque put into it. Generally, the mechanical advantage is calculated as follows:
MA = \frac{\text{distance over which effort is applied}}{\text{distance over which the load is moved}}
or more simply:
MA = \frac{\text{output force}}{\text{input force}}
The first equation shows that the force exerted IN to the machine multiplied by the distance moved IN will always be equal to the force exerted OUT of the machine multiplied by the distance moved OUT. For example, using a block and tackle with 6 ropes, and a 600 pound load, the operator would be required to pull the rope 6 feet, and exert 100 pounds of force to lift the 600 pound load 1 foot.
The second equation is a simplified formula based just on the forces in and out. Using the example above, 100 pounds of force IN results in 600 pounds of force OUT, an MA of 6. Both of these equations calculate only the ideal mechanical advantage (IMA) and ignore any losses due to friction.
The actual mechanical advantage (AMA) includes those frictional losses. The difference between the two is the mechanical efficiency of the system.
Contents
[hide]
* 1 Simple machines
o 1.1 Pulleys
o 1.2 Screws
* 2 Types
o 2.1 Ideal mechanical advantage
o 2.2 Actual mechanical advantage
* 3 See also
* 4 References
o 4.1 Notes
o 4.2 Bibliography
* 5 External links
[edit] Simple machines
Beam balanced around a fulcrum
The following simple machines exhibit a mechanical advantage:
* The beam shown is in static equilibrium around the fulcrum. This is due to the moment created by vector force "A" counterclockwise (moment A*a) being in equilibrium with the moment created by vector force "B" clockwise (moment B*b). The relatively low vector force "B" is translated in a relatively high vector force "A". The force is thus increased in the ratio of the forces A : B, which is equal to the ratio of the distances to the fulcrum b : a. This ratio is called the mechanical advantage. This idealised situation does not take into account friction. For more explanation, see also lever.
* Wheel and axle notion (e.g. screwdrivers, doorknobs): A wheel is essentially a lever with one arm the distance between the axle and the outer point of the wheel, and the other the radius of the axle. Typically this is a fairly large difference, leading to a proportionately large mechanical advantage. This allows even simple wheels with wooden axles running in wooden blocks to still turn freely, because their friction is overwhelmed by the rotational force of the wheel multiplied by the mechanical advantage.
* Pulley: Pulleys change the direction of a tension force on a flexible material, e.g. a rope or cable. In addition, pulleys can be "added together" to create mechanical advantage, by having the flexible material looped over several pulleys in turn. Adding more loops and pulleys increases the mechanical advantage.
* Screw: A screw is essentially an inclined plane wrapped around a cylinder. The run over the rise of this inclined plane is the mechanical advantage of a screw.[1]
[edit] Pulleys
An example of a rope and pulley system illustrating mechanical advantage.
Consider lifting a weight with rope and pulleys. A rope looped through a pulley attached to a fixed spot, e.g. a barn roof rafter, and attached to the weight is called a single fixed pulley. It has an MA = 1 (assuming frictionless bearings in the pulley), meaning no mechanical advantage (or disadvantage) however advantageous the change in direction may be.
A single movable pulley has an MA of 2 (assuming frictionless bearings in the pulley). Consider a pulley attached to a weight being lifted. A rope passes around it, with one end attached to a fixed point above, e.g. a barn roof rafter, and a pulling force is applied upward to the other end with the two lengths parallel. In this situation the distance the lifter must pull the rope becomes twice the distance the weight travels, allowing the force applied to be halved. Note: if an additional pulley is used to change the direction of the rope, e.g. the person doing the work wants to stand on the ground instead of on a rafter, the mechanical advantage is not increased.
By looping more ropes around more pulleys we can continue to increase the mechanical advantage. For example if we have two pulleys attached to the rafter, two pulleys attached to the weight, one end attached to the rafter, and someone standing on the rafter pulling the rope, we have a mechanical advantage of four. Again note: if we add another pulley so that someone may stand on the ground and pull down, we still have a mechanical advantage of four.
Here are examples where the fixed point is not obvious:
* A velcro strap on a shoe passes through a slot and folds over on itself. The slot is a movable pulley and the MA = 2.
* Two ropes laid down a ramp attached to a raised platform. A barrel is rolled onto the ropes and the ropes are passed over the barrel and handed to two workers at the top of the ramp. The workers pull the ropes together to get the barrel to the top. The barrel is a movable pulley and the MA = 2. If there is enough friction where the rope is pinched between the barrel and the ramp, the pinch point becomes the attachment point. This is considered a fixed attachment point because the rope above the barrel does not move relative to the ramp. Alternatively the ends of the rope can be attached to the platform.
* Block and tackle: MA = 3
* Inclined plane: MA = length of slope ÷ height of slope
[edit] Screws
The theoretical mechanical advantage for a screw can be calculated using the following equation:[2]
MA = \frac{\pi d_m}{l}
where
dm = the mean diameter of the screw thread
l = the lead of the screw thread
Note that the actual mechanical advantage of a screw system is greater, as a screwdriver or other screw driving system has a mechanical advantage as well.
[edit] Types
There are two types of mechanical advantage:
1. Ideal mechanical advantage (IMA)
2. Actual mechanical advantage (AMA)
[edit] Ideal mechanical advantage
The ideal mechanical advantage (IMA), or theoretical mechanical advantage, is the mechanical advantage of an ideal machine. It is usually calculated using physics principles because there is no ideal machine.
The IMA of a machine can be found with the following formula:
IMA = \frac {D_E} {D_R}
where
DE equals the effort distance (the distance from the fulcrum to where the effort is applied)
DR equals the resistance distance (the distance from the fulcrum to where the resistance is applied)
[edit] Actual mechanical advantage
The actual mechanical advantage (AMA) is the mechanical advantage of a real machine. Actual mechanical advantage takes into consideration real world factors such as energy lost in friction.
The AMA of a machine is calculated with the following formula:
AMA = \frac {R} {E_\text{actual}}
where
R = resistance force
Eactual = actual effort force
♣☺☻puLley☻☺♣
Pulley
From Wikipedia, the free encyclopedia
Jump to: navigation, search
For the band, see Pulley (band). For the village, see Pulley, Shropshire. For the American photographer, see Gerald P. Pulley.
Pulleys on a ship. In this context, pulleys are usually known as blocks.
A pulley (also called a block) is a mechanism composed of a wheel (called a sheave) with a groove between two flanges around the wheel's circumference. A rope, cable or belt usually runs inside the groove. Pulleys are used to change the direction of an applied force, transmit rotational motion, or realize a mechanical advantage in either a linear or rotational system of motion.
Contents
[hide]
* 1 Belt and pulley systems
* 2 Rope and pulley systems
o 2.1 Types of systems
o 2.2 How it works
* 3 See also
[edit] Belt and pulley systems
Belt and pulley system
A belt and pulley system is characterized by two or more pulleys in common to a belt. This allows for mechanical power, torque, and speed to be transmitted across axes and, if the pulleys are of differing diameters, a mechanical advantage to be realized.
A belt drive is analogous to that of a chain drive, however a belt sheave may be smooth (devoid of discrete interlocking members as would be found on a chain sprocket, spur gear, or timing belt) so that the mechanical advantage is given by the ratio of the pitch diameter of the sheaves only (one is not able to count 'teeth' to determine gear ratio).
[edit] Rope and pulley systems
Also called block and tackles, rope and pulley systems (the rope may be a light line or a strong cable) are characterized by the use of one rope transmitting a linear motive force (in tension) to a load through one or more pulleys for the purpose of pulling the load (often against gravity.) They are often included in the list of simple machines.
In a system of a single rope and pulleys, when friction is neglected, the mechanical advantage gained can be calculated by counting the number of rope lengths exerting force on the load. Since the tension in each rope length is equal to the force exerted on the free end of the rope, the mechanical advantage is simply equal to the number of ropes pulling on the load. For example, in Diagram 3 below, there is one rope attached to the load, and 2 rope lengths extending from the pulley attached to the load, for a total of 3 ropes supporting it. If the force applied to the free end of the rope is 10 lb, each of these rope lengths will exert a force of 10 lb. on the load, for a total of 30 lb. So the mechanical advantage is 3.
The force on the load is increased by the mechanical advantage; however the distance the load moves, compared to the length the free end of the rope moves, is decreased in the same proportion. Since a slender cable is more easily managed than a fat one (albeit shorter and stronger), pulley systems are often the preferred method of applying mechanical advantage to the pulling force of a winch (as can be found in a lift crane).
Pulley systems are the only simple machines in which the possible values of mechanical advantage are limited to whole numbers.
In practice, the more pulleys there are, the less efficient a system is. This is due to sliding friction in the system where cable meets pulley and in the rotational mechanism of each pulley.
It is not recorded when or by whom the pulley was first developed. It is believed however that Archimedes developed the first documented block and tackle pulley system, as recorded by Plutarch. Plutarch reported that Archimedes moved an entire warship, laden with men, using compound pulleys and his own strength.
[edit] Types of systems
Fixed pulley
Movable pulley
These are different types of pulley systems:
* Fixed A fixed or class 1 pulley has a fixed axle. That is, the axle is "fixed" or anchored in place. A fixed pulley is used to change the direction of the force on a rope (called a belt). A fixed pulley has a mechanical advantage of 1. A mechanical advantage of one means that the force is equal on both sides of the pulley and there is no multiplication of force.
* Movable A movable or class 2 pulley has a free axle. That is, the axle is "free" to move in space. A movable pulley is used to multiply forces. A movable pulley has a mechanical advantage of 2. That is, if one end of the rope is anchored, pulling on the other end of the rope will apply a doubled force to the object attached to the pulley.
* Compound A compound pulley is a combination of a fixed and a movable pulley system.
o Block and tackle - A block and tackle is a compound pulley where several pulleys are mounted on each axle, further increasing the mechanical advantage. Block and tackles usually lift objects with a mechanical advantage greater than 2.
[edit] How it works
Diagram 1 - A basic equation for a pulley: In equilibrium, the force F on the pulley axle is equal and opposite to the sum of the tensions in each line leaving the pulley, and these tensions are equal.
Diagram 2 - A simple pulley system - a single movable pulley lifting weight W. The tension in each line is W/2, yielding an advantage of 2.
Diagram 2a - Another simple pulley system similar to diagram 2, but in which the lifting force is redirected downward.
A practical compound pulley corresponding to diagram 2a.
The simplest theory of operation for a pulley system assumes that the pulleys and lines are weightless, and that there is no energy loss due to friction. It is also assumed that the lines do not stretch.
A crane using the compound pulley system yielding an advantage of 4. The single fixed pulley is installed on the crane. The two movable pulleys (joined together) are attached to the hook. One end of the rope is attached to the crane frame, another - to the winch.
In equilibrium, the total force on the pulley must be zero. This means that the force on the axle of the pulley is shared equally by the two lines looping through the pulley. The situation is schematically illustrated in diagram 1. For the case where the lines are not parallel, the tensions in each line are still equal, but now the vector sum of all forces is zero.
A second basic equation for the pulley follows from the conservation of energy: The product of the weight lifted times the distance it is moved is equal to the product of the lifting force (the tension in the lifting line) times the distance the lifting line is moved. The weight lifted divided by the lifting force is defined as the advantage of the pulley system.
It is important to notice that a system of pulleys does not change the amount of work done. The work is given by the force times the distance moved. The pulley simply allows trading force for distance: you pull with less force, but over a longer distance.
In diagram 2, a single movable pulley allows weight W to be lifted with only half the force needed to lift the weight without assistance. The total force needed is divided between the lifting force (red arrow) and the "ceiling" which is some immovable object (such as the earth). In this simple system, the lifting force is directed in the same direction as the movement of the weight. The advantage of this system is 2. Although the force needed to lift the weight is only W/2, we will need to draw a length of rope that is twice the distance that the weight is lifted, so that the total amount of work done (Force x distance) remains the same.
A second pulley may be added as in diagram 2a, which simply serves to redirect the lifting force downward, it does not change the advantage of the system.
Diagram 3 - A simple compound pulley system - a movable pulley and a fixed pulley lifting weight W. The tension in each line is one W/3, yielding an advantage of 3.
Diagram 3a - A simple compound pulley system - a movable pulley and a fixed pulley lifting weight W, with an additional pulley redirecting the lifting force downward. The tension in each line is one W/3, yielding an advantage of 3.
Diagram 4a - A more complicated compound pulley system. The tension in each line is W/4, yielding an advantage of 4. An additional pulley redirecting the lifting force has been added.
Figure 4b - A practical block and tackle pulley system corresponding to diagram 4a. Note that the axles of the fixed and movable pulleys have been combined.
The addition of a fixed pulley to the single pulley system can yield an increase of advantage. In diagram 3, the addition of a fixed pulley yields a lifting advantage of 3. The tension in each line is W/3, and the force on the axles of each pulley is 2W/3. As in the case of diagram 2a, another pulley may be added to reverse the direction of the lifting force, but with no increase in advantage. This situation is shown in diagram 3a.
This process can be continued indefinitely for ideal pulleys with each additional pulley yielding a unit increase in advantage. For real pulleys friction among rope and pulleys will increase as more pulleys are added to the point that no advantage is possible. It puts a limit for the number of pulleys usable in practice. The above pulley systems are known collectively as block and tackle pulley systems. In diagram 4a, a block and tackle system with advantage 4 is shown. A practical implementation in which the connection to the ceiling is combined and the fixed and movable pulleys are encased in single housings is shown in figure 4b.
Other pulley systems are possible, and some can deliver an increased advantage with fewer pulleys than the block and tackle system. The advantage of the block and tackle system is that each pulley and line is subjected to equal tensions and forces. Efficient design dictates that each line and pulley be capable of handling its load, and no more. Other pulley designs will require different strengths of line and pulleys depending on their position in the system, but a block and tackle system can use the same line size throughout, and can mount the fixed and movable pulleys on a common axle.
From Wikipedia, the free encyclopedia
Jump to: navigation, search
For the band, see Pulley (band). For the village, see Pulley, Shropshire. For the American photographer, see Gerald P. Pulley.
Pulleys on a ship. In this context, pulleys are usually known as blocks.
A pulley (also called a block) is a mechanism composed of a wheel (called a sheave) with a groove between two flanges around the wheel's circumference. A rope, cable or belt usually runs inside the groove. Pulleys are used to change the direction of an applied force, transmit rotational motion, or realize a mechanical advantage in either a linear or rotational system of motion.
Contents
[hide]
* 1 Belt and pulley systems
* 2 Rope and pulley systems
o 2.1 Types of systems
o 2.2 How it works
* 3 See also
[edit] Belt and pulley systems
Belt and pulley system
A belt and pulley system is characterized by two or more pulleys in common to a belt. This allows for mechanical power, torque, and speed to be transmitted across axes and, if the pulleys are of differing diameters, a mechanical advantage to be realized.
A belt drive is analogous to that of a chain drive, however a belt sheave may be smooth (devoid of discrete interlocking members as would be found on a chain sprocket, spur gear, or timing belt) so that the mechanical advantage is given by the ratio of the pitch diameter of the sheaves only (one is not able to count 'teeth' to determine gear ratio).
[edit] Rope and pulley systems
Also called block and tackles, rope and pulley systems (the rope may be a light line or a strong cable) are characterized by the use of one rope transmitting a linear motive force (in tension) to a load through one or more pulleys for the purpose of pulling the load (often against gravity.) They are often included in the list of simple machines.
In a system of a single rope and pulleys, when friction is neglected, the mechanical advantage gained can be calculated by counting the number of rope lengths exerting force on the load. Since the tension in each rope length is equal to the force exerted on the free end of the rope, the mechanical advantage is simply equal to the number of ropes pulling on the load. For example, in Diagram 3 below, there is one rope attached to the load, and 2 rope lengths extending from the pulley attached to the load, for a total of 3 ropes supporting it. If the force applied to the free end of the rope is 10 lb, each of these rope lengths will exert a force of 10 lb. on the load, for a total of 30 lb. So the mechanical advantage is 3.
The force on the load is increased by the mechanical advantage; however the distance the load moves, compared to the length the free end of the rope moves, is decreased in the same proportion. Since a slender cable is more easily managed than a fat one (albeit shorter and stronger), pulley systems are often the preferred method of applying mechanical advantage to the pulling force of a winch (as can be found in a lift crane).
Pulley systems are the only simple machines in which the possible values of mechanical advantage are limited to whole numbers.
In practice, the more pulleys there are, the less efficient a system is. This is due to sliding friction in the system where cable meets pulley and in the rotational mechanism of each pulley.
It is not recorded when or by whom the pulley was first developed. It is believed however that Archimedes developed the first documented block and tackle pulley system, as recorded by Plutarch. Plutarch reported that Archimedes moved an entire warship, laden with men, using compound pulleys and his own strength.
[edit] Types of systems
Fixed pulley
Movable pulley
These are different types of pulley systems:
* Fixed A fixed or class 1 pulley has a fixed axle. That is, the axle is "fixed" or anchored in place. A fixed pulley is used to change the direction of the force on a rope (called a belt). A fixed pulley has a mechanical advantage of 1. A mechanical advantage of one means that the force is equal on both sides of the pulley and there is no multiplication of force.
* Movable A movable or class 2 pulley has a free axle. That is, the axle is "free" to move in space. A movable pulley is used to multiply forces. A movable pulley has a mechanical advantage of 2. That is, if one end of the rope is anchored, pulling on the other end of the rope will apply a doubled force to the object attached to the pulley.
* Compound A compound pulley is a combination of a fixed and a movable pulley system.
o Block and tackle - A block and tackle is a compound pulley where several pulleys are mounted on each axle, further increasing the mechanical advantage. Block and tackles usually lift objects with a mechanical advantage greater than 2.
[edit] How it works
Diagram 1 - A basic equation for a pulley: In equilibrium, the force F on the pulley axle is equal and opposite to the sum of the tensions in each line leaving the pulley, and these tensions are equal.
Diagram 2 - A simple pulley system - a single movable pulley lifting weight W. The tension in each line is W/2, yielding an advantage of 2.
Diagram 2a - Another simple pulley system similar to diagram 2, but in which the lifting force is redirected downward.
A practical compound pulley corresponding to diagram 2a.
The simplest theory of operation for a pulley system assumes that the pulleys and lines are weightless, and that there is no energy loss due to friction. It is also assumed that the lines do not stretch.
A crane using the compound pulley system yielding an advantage of 4. The single fixed pulley is installed on the crane. The two movable pulleys (joined together) are attached to the hook. One end of the rope is attached to the crane frame, another - to the winch.
In equilibrium, the total force on the pulley must be zero. This means that the force on the axle of the pulley is shared equally by the two lines looping through the pulley. The situation is schematically illustrated in diagram 1. For the case where the lines are not parallel, the tensions in each line are still equal, but now the vector sum of all forces is zero.
A second basic equation for the pulley follows from the conservation of energy: The product of the weight lifted times the distance it is moved is equal to the product of the lifting force (the tension in the lifting line) times the distance the lifting line is moved. The weight lifted divided by the lifting force is defined as the advantage of the pulley system.
It is important to notice that a system of pulleys does not change the amount of work done. The work is given by the force times the distance moved. The pulley simply allows trading force for distance: you pull with less force, but over a longer distance.
In diagram 2, a single movable pulley allows weight W to be lifted with only half the force needed to lift the weight without assistance. The total force needed is divided between the lifting force (red arrow) and the "ceiling" which is some immovable object (such as the earth). In this simple system, the lifting force is directed in the same direction as the movement of the weight. The advantage of this system is 2. Although the force needed to lift the weight is only W/2, we will need to draw a length of rope that is twice the distance that the weight is lifted, so that the total amount of work done (Force x distance) remains the same.
A second pulley may be added as in diagram 2a, which simply serves to redirect the lifting force downward, it does not change the advantage of the system.
Diagram 3 - A simple compound pulley system - a movable pulley and a fixed pulley lifting weight W. The tension in each line is one W/3, yielding an advantage of 3.
Diagram 3a - A simple compound pulley system - a movable pulley and a fixed pulley lifting weight W, with an additional pulley redirecting the lifting force downward. The tension in each line is one W/3, yielding an advantage of 3.
Diagram 4a - A more complicated compound pulley system. The tension in each line is W/4, yielding an advantage of 4. An additional pulley redirecting the lifting force has been added.
Figure 4b - A practical block and tackle pulley system corresponding to diagram 4a. Note that the axles of the fixed and movable pulleys have been combined.
The addition of a fixed pulley to the single pulley system can yield an increase of advantage. In diagram 3, the addition of a fixed pulley yields a lifting advantage of 3. The tension in each line is W/3, and the force on the axles of each pulley is 2W/3. As in the case of diagram 2a, another pulley may be added to reverse the direction of the lifting force, but with no increase in advantage. This situation is shown in diagram 3a.
This process can be continued indefinitely for ideal pulleys with each additional pulley yielding a unit increase in advantage. For real pulleys friction among rope and pulleys will increase as more pulleys are added to the point that no advantage is possible. It puts a limit for the number of pulleys usable in practice. The above pulley systems are known collectively as block and tackle pulley systems. In diagram 4a, a block and tackle system with advantage 4 is shown. A practical implementation in which the connection to the ceiling is combined and the fixed and movable pulleys are encased in single housings is shown in figure 4b.
Other pulley systems are possible, and some can deliver an increased advantage with fewer pulleys than the block and tackle system. The advantage of the block and tackle system is that each pulley and line is subjected to equal tensions and forces. Efficient design dictates that each line and pulley be capable of handling its load, and no more. Other pulley designs will require different strengths of line and pulleys depending on their position in the system, but a block and tackle system can use the same line size throughout, and can mount the fixed and movable pulleys on a common axle.
♣♦☺f0rcE mult!pliers☺♦♣
Force multiplication
From Wikipedia, the free encyclopedia
(Redirected from Force multiplier)
Jump to: navigation, search
Force multiplication, in military usage, refers to a combination of attributes or advantages which make a given force more effective than another force of comparable size. A force multiplier refers to a factor that dramatically increases (hence "multiplies") the effectiveness of an item or group.
Some common force multipliers are:
* Morale
* Technology
* Geographical features
* Weather
* Recruitment through diplomacy
* Training and experience
* Fearsome reputation
* Deception
Some factors may influence one another, e.g. enhanced technology improving morale or geographical features allowing deception.
It seems clear that force multiplication existed before anyone had a name for it. While the Mongols used swarming tactics coordinated by non-electronic communications, such primitive tactics nevertheless made them notably effective. To protect the archers at the Battle of Agincourt, stakes were driven into the ground. This is an example of "combined arms," a doctrinal development and another example of force multiplication.
When WWI aviators first greeted their opponents with friendly waves, no one realized the multiplicative effect of tactical air reconnaissance. However, after the command on both sides became aware of how powerful it could be, aviators started shooting at each other. At first they did so with rifles and then with purpose-built aircraft guns.
Contents
[hide]
* 1 Doctrinal changes
* 2 Psychology
* 3 Simplicity
* 4 Technology
o 4.1 Bombers
o 4.2 Fighter combat
* 5 Creating local forces
* 6 Deception
* 7 See also
* 8 References
[edit] Doctrinal changes
In the First World War, the Germans experimented with what were called "storm tactics," where a small group of highly trained soldiers (stormtroopers) would open a salient through which much larger forces could penetrate. This met with only limited success, while the 1939 Blitzkrieg, which broke through with coordinated mechanized ground forces with aircraft in close support, was vastly more effective.
Towards the end of WWII, the German army introduced kampfgruppe combat formations that were composed of whatever units happened to be available. Though poor quality ones generally constituted the major part of them, they often performed successfully because of their high degree of flexibility and adaptability. Mission-type tactics, as opposed to extremely specific directives that give no discretion to the junior commander, are widely used by modern militaries now due to their force multiplication. Originating from German concepts of Auftragstaktik, these tactics may be developing even more rapidly in the concept of network-centric warfare, where subordinate commanders receive information not only from their own commanders, but from adjacent units.
A different paradigm was one of the results of the theories of John Boyd, the "high-low mix" in which a large number of less expensive aircraft, coupled with a small number of extremely capable "silver bullet" aircraft, had the effect of a much larger force. Boyd's concept of quick action is based on the repeated application of the Boyd loop, consisting of the steps
* Observe: make use of the best sensors and other intelligence available
* Orient: put the new observations into a context with the old
* Decide: select the next action based on the combined observation and local knowledge
* Act: carry out the selected action, ideally while the opponent is still observing your last action.
Boyd's concept is also known as the OODA Loop, and is a description of the decision-making process that Boyd contended applies to business, sports, law enforcement and military operations. Boyd's doctrine is widely taught in the American military, and one of the aims of network centric warfare is to get inside his OODA loop--that is, to go from observation to action before the enemy can get past orientation, preventing him from ever being able to make an effective decision or put it into action. Small unit leadership is critical to this, and NCW's ability to disseminate information to small unit leaders enables such tactics.
Network-centric warfare can provide additional information and can help prevent fratricide, but also allows swarm tactics [1] and the seizing of opportunities by subordinate forces. These are a realization of Boyd's theories. (Rand-Edwards-2000 pg. 2) defines " "a swarming case is any historical example in which the scheme of maneuver involves the convergent attack of five (or more) semiautonomous (or autonomous) units on a targeted force in some particular place. "Convergent" implies an attack from most of the points on the compass."
Another version of "swarming" is evident in air-to-ground attack formations in which the attack aircraft do not approach from one direction, at one time, or at the same altitude, but schedule the attacks so each one requires a Boyd-style OODA iteration to deal with a new threat [2]. Replacement training units (RTU) were "finishing schools" for pilots that needed to know not just the school solution, but the actual tactics being used in Vietnam. Referring to close air support, "In the RTU, new pilots learned the rules of the road for working with a Forward Air Controller (FAC). The hardest part was finding the small aircraft as it circled over the target area. The fast-moving fighters used directional finding/steering equipment to get close enough to the slow, low FAC until someone in the flight could get an eyeball on him—a tally-ho. Once the FAC was in sight, he would give the fighters a target briefing—type of target, elevation, attack heading, location of friendlies, enemy defensive fire, best egress heading if hit by enemy fire, and other pertinent data. Usually the fighters would set up a circle, called a wheel or wagon wheel, over the FAC, and wait for him to mark the target. Once the target was marked, the flight leader would attack first.
[edit] Psychology
Napoleon is well known for his comment "The moral is to the physical as three to one [3]." Former Secretary of State and Chairman of the Joint Chiefs of Staff Colin Powell has said: "Perpetual optimism is a force multiplier." [4] Morale, training, and ethos have long been known to result in disproportionate effects on the battlefield. A volunteer military is, soldier for soldier, significantly more effective than a conscription force.
Psychological Warfare can target the morale, politics, and values of enemy soldiers and their supporters to effectively neutralize them in a conflict.
[edit] Simplicity
A small group of well-equipped, well-trained soldiers with the sun at their backs may be more capable of defending a fortified mountainous position against a larger group of poorly equipped, poorly-trained soldiers with the sun in their faces. Careful selection of the battle site, which is often considered operational art, clearly multiplies the effectiveness of the force that selects it.
A given infantry division may be able to advance 12 miles (20 km) in a day. Assigning trucks to this division will act as a force multiplier allowing the division to advance 40 miles (65 km) in a day. Air support, artillery, and other specialized weapons systems are other examples of factors that may dramatically increase the division's capabilities. In a similar vein, air-to-air refueling tankers employed by some Air Forces throughout the world are massive force multipliers with aircraft - negating having to travel all the way back to a military base or carrier in order to refuel on a frequent basis - thus extending the ranges of fighters and bombers, keeping fighters over their targets or on Combat Air Patrols for significantly more time.
[edit] Technology
In the First World War, there were two abortive experiments where, had the high commands had the imagination to realize the potential use of new weapons, there could have been a massive breakthrough through the stalemate of trench warfare. The first was the large-scale German use of chemical weapons at the Second Battle of Ypres, and the second was the large-scale British use of tanks at the Battle of Cambrai in 1917. Either of these new attack methods could have opened an enormous breach in the enemy lines, but failed, as did the Battle of the Crater in the American Civil War.
[edit] Bombers
At one extreme, a stealthy aircraft can attack a target without needing the large numbers of escort fighters, electronic warfare, air defense suppression, and other supporting aircraft that would be needed were conventional bombers used against the same target.
Whether or not the aircraft have low observability, precision guided munitions (PGM) give an immense multiplication. The Thanh Hoa Bridge in North Vietnam had been only mildly damaged by approximately 800 sorties by aircraft armed with conventional "dumb" bombs, but had one of its spans destroyed by a 12-plane mission, of which 8 carried laser-guided bombs. Two small subsequent missions, again with laser-guided bombs, completed the destruction of this target. Precision guided munitions are one example of what has been called the Revolution in Military Affairs. In WWII, British night bombers could hit, at best, an area of a city.
Modern PGMs commonly put a bomb within 3-10 meters of its target. See the use of heavy bombers in direct support of friendly troops in Afghanistan, using the technique of Ground-Aided Precision Strike.
[edit] Fighter combat
Fighter aircraft coordinated by an AWACS control aircraft, so that they can approach targets without being revealed by their own radar, and who are assigned to take specific targets so that duplication is avoided, is far more effective than an equivalent number of fighters dependent on their own resources for target acquisition.
In exercises between the Indian and US air forces, the Indian pilots had an opportunity to operate with AWACS control, and found it extremely effective [5]. India has ordered AWACS aircraft, using Israeli Phalcon electronics on a Russian airframe, and this exercise is part of their preparation. Officer and pilot comments included "definitely was a force multiplier. Giving you an eye deep beyond you"... "We could pick up incoming targets whether aircraft or missiles almost 400 kilometers away. It gives a grand battle coordination in the air".
[edit] Creating local forces
The use of small numbers of specialists to create larger effective forces is another form of multiplication. The basic A Team of US Army Special Forces is a 12-man unit that can train and lead a company-sized unit (100-200 men) of local guerrillas. While it is not clear when the term "force multiplier" first appeared in the military literature, the use of small teams to raise much larger guerrilla units was among the first uses of the term.
[edit] Deception
Deception can produce the potential effect of a much larger force. The fictitious First United States Army Group (FUSAG) was portrayed to the WWII Germans as the main force for the invasion of Europe. Operation Bodyguard [6] successfully gave the impression that FUSAG was to land at the Pas de Calais, convincing the Germans that the real attack at Normandy was a feint. As a result of the successful deception, the Normandy force penetrated deeply, in part, because the Germans held back strategic reserves that they thought would be needed at the Pas de Calais, against what was a nonexistent force. FUSAG's existence was suggested by the use of decoy vehicles that the Allies allowed to be photographed, fictitious radio traffic generated by a small number of specialists, and the Double Cross System [7]. Double Cross referred to turning all surviving German spies in the UK into double agents, who sent back convincing reports that were consistent with the deception programs being conducted by the London Controlling Section.
From Wikipedia, the free encyclopedia
(Redirected from Force multiplier)
Jump to: navigation, search
Force multiplication, in military usage, refers to a combination of attributes or advantages which make a given force more effective than another force of comparable size. A force multiplier refers to a factor that dramatically increases (hence "multiplies") the effectiveness of an item or group.
Some common force multipliers are:
* Morale
* Technology
* Geographical features
* Weather
* Recruitment through diplomacy
* Training and experience
* Fearsome reputation
* Deception
Some factors may influence one another, e.g. enhanced technology improving morale or geographical features allowing deception.
It seems clear that force multiplication existed before anyone had a name for it. While the Mongols used swarming tactics coordinated by non-electronic communications, such primitive tactics nevertheless made them notably effective. To protect the archers at the Battle of Agincourt, stakes were driven into the ground. This is an example of "combined arms," a doctrinal development and another example of force multiplication.
When WWI aviators first greeted their opponents with friendly waves, no one realized the multiplicative effect of tactical air reconnaissance. However, after the command on both sides became aware of how powerful it could be, aviators started shooting at each other. At first they did so with rifles and then with purpose-built aircraft guns.
Contents
[hide]
* 1 Doctrinal changes
* 2 Psychology
* 3 Simplicity
* 4 Technology
o 4.1 Bombers
o 4.2 Fighter combat
* 5 Creating local forces
* 6 Deception
* 7 See also
* 8 References
[edit] Doctrinal changes
In the First World War, the Germans experimented with what were called "storm tactics," where a small group of highly trained soldiers (stormtroopers) would open a salient through which much larger forces could penetrate. This met with only limited success, while the 1939 Blitzkrieg, which broke through with coordinated mechanized ground forces with aircraft in close support, was vastly more effective.
Towards the end of WWII, the German army introduced kampfgruppe combat formations that were composed of whatever units happened to be available. Though poor quality ones generally constituted the major part of them, they often performed successfully because of their high degree of flexibility and adaptability. Mission-type tactics, as opposed to extremely specific directives that give no discretion to the junior commander, are widely used by modern militaries now due to their force multiplication. Originating from German concepts of Auftragstaktik, these tactics may be developing even more rapidly in the concept of network-centric warfare, where subordinate commanders receive information not only from their own commanders, but from adjacent units.
A different paradigm was one of the results of the theories of John Boyd, the "high-low mix" in which a large number of less expensive aircraft, coupled with a small number of extremely capable "silver bullet" aircraft, had the effect of a much larger force. Boyd's concept of quick action is based on the repeated application of the Boyd loop, consisting of the steps
* Observe: make use of the best sensors and other intelligence available
* Orient: put the new observations into a context with the old
* Decide: select the next action based on the combined observation and local knowledge
* Act: carry out the selected action, ideally while the opponent is still observing your last action.
Boyd's concept is also known as the OODA Loop, and is a description of the decision-making process that Boyd contended applies to business, sports, law enforcement and military operations. Boyd's doctrine is widely taught in the American military, and one of the aims of network centric warfare is to get inside his OODA loop--that is, to go from observation to action before the enemy can get past orientation, preventing him from ever being able to make an effective decision or put it into action. Small unit leadership is critical to this, and NCW's ability to disseminate information to small unit leaders enables such tactics.
Network-centric warfare can provide additional information and can help prevent fratricide, but also allows swarm tactics [1] and the seizing of opportunities by subordinate forces. These are a realization of Boyd's theories. (Rand-Edwards-2000 pg. 2) defines " "a swarming case is any historical example in which the scheme of maneuver involves the convergent attack of five (or more) semiautonomous (or autonomous) units on a targeted force in some particular place. "Convergent" implies an attack from most of the points on the compass."
Another version of "swarming" is evident in air-to-ground attack formations in which the attack aircraft do not approach from one direction, at one time, or at the same altitude, but schedule the attacks so each one requires a Boyd-style OODA iteration to deal with a new threat [2]. Replacement training units (RTU) were "finishing schools" for pilots that needed to know not just the school solution, but the actual tactics being used in Vietnam. Referring to close air support, "In the RTU, new pilots learned the rules of the road for working with a Forward Air Controller (FAC). The hardest part was finding the small aircraft as it circled over the target area. The fast-moving fighters used directional finding/steering equipment to get close enough to the slow, low FAC until someone in the flight could get an eyeball on him—a tally-ho. Once the FAC was in sight, he would give the fighters a target briefing—type of target, elevation, attack heading, location of friendlies, enemy defensive fire, best egress heading if hit by enemy fire, and other pertinent data. Usually the fighters would set up a circle, called a wheel or wagon wheel, over the FAC, and wait for him to mark the target. Once the target was marked, the flight leader would attack first.
[edit] Psychology
Napoleon is well known for his comment "The moral is to the physical as three to one [3]." Former Secretary of State and Chairman of the Joint Chiefs of Staff Colin Powell has said: "Perpetual optimism is a force multiplier." [4] Morale, training, and ethos have long been known to result in disproportionate effects on the battlefield. A volunteer military is, soldier for soldier, significantly more effective than a conscription force.
Psychological Warfare can target the morale, politics, and values of enemy soldiers and their supporters to effectively neutralize them in a conflict.
[edit] Simplicity
A small group of well-equipped, well-trained soldiers with the sun at their backs may be more capable of defending a fortified mountainous position against a larger group of poorly equipped, poorly-trained soldiers with the sun in their faces. Careful selection of the battle site, which is often considered operational art, clearly multiplies the effectiveness of the force that selects it.
A given infantry division may be able to advance 12 miles (20 km) in a day. Assigning trucks to this division will act as a force multiplier allowing the division to advance 40 miles (65 km) in a day. Air support, artillery, and other specialized weapons systems are other examples of factors that may dramatically increase the division's capabilities. In a similar vein, air-to-air refueling tankers employed by some Air Forces throughout the world are massive force multipliers with aircraft - negating having to travel all the way back to a military base or carrier in order to refuel on a frequent basis - thus extending the ranges of fighters and bombers, keeping fighters over their targets or on Combat Air Patrols for significantly more time.
[edit] Technology
In the First World War, there were two abortive experiments where, had the high commands had the imagination to realize the potential use of new weapons, there could have been a massive breakthrough through the stalemate of trench warfare. The first was the large-scale German use of chemical weapons at the Second Battle of Ypres, and the second was the large-scale British use of tanks at the Battle of Cambrai in 1917. Either of these new attack methods could have opened an enormous breach in the enemy lines, but failed, as did the Battle of the Crater in the American Civil War.
[edit] Bombers
At one extreme, a stealthy aircraft can attack a target without needing the large numbers of escort fighters, electronic warfare, air defense suppression, and other supporting aircraft that would be needed were conventional bombers used against the same target.
Whether or not the aircraft have low observability, precision guided munitions (PGM) give an immense multiplication. The Thanh Hoa Bridge in North Vietnam had been only mildly damaged by approximately 800 sorties by aircraft armed with conventional "dumb" bombs, but had one of its spans destroyed by a 12-plane mission, of which 8 carried laser-guided bombs. Two small subsequent missions, again with laser-guided bombs, completed the destruction of this target. Precision guided munitions are one example of what has been called the Revolution in Military Affairs. In WWII, British night bombers could hit, at best, an area of a city.
Modern PGMs commonly put a bomb within 3-10 meters of its target. See the use of heavy bombers in direct support of friendly troops in Afghanistan, using the technique of Ground-Aided Precision Strike.
[edit] Fighter combat
Fighter aircraft coordinated by an AWACS control aircraft, so that they can approach targets without being revealed by their own radar, and who are assigned to take specific targets so that duplication is avoided, is far more effective than an equivalent number of fighters dependent on their own resources for target acquisition.
In exercises between the Indian and US air forces, the Indian pilots had an opportunity to operate with AWACS control, and found it extremely effective [5]. India has ordered AWACS aircraft, using Israeli Phalcon electronics on a Russian airframe, and this exercise is part of their preparation. Officer and pilot comments included "definitely was a force multiplier. Giving you an eye deep beyond you"... "We could pick up incoming targets whether aircraft or missiles almost 400 kilometers away. It gives a grand battle coordination in the air".
[edit] Creating local forces
The use of small numbers of specialists to create larger effective forces is another form of multiplication. The basic A Team of US Army Special Forces is a 12-man unit that can train and lead a company-sized unit (100-200 men) of local guerrillas. While it is not clear when the term "force multiplier" first appeared in the military literature, the use of small teams to raise much larger guerrilla units was among the first uses of the term.
[edit] Deception
Deception can produce the potential effect of a much larger force. The fictitious First United States Army Group (FUSAG) was portrayed to the WWII Germans as the main force for the invasion of Europe. Operation Bodyguard [6] successfully gave the impression that FUSAG was to land at the Pas de Calais, convincing the Germans that the real attack at Normandy was a feint. As a result of the successful deception, the Normandy force penetrated deeply, in part, because the Germans held back strategic reserves that they thought would be needed at the Pas de Calais, against what was a nonexistent force. FUSAG's existence was suggested by the use of decoy vehicles that the Allies allowed to be photographed, fictitious radio traffic generated by a small number of specialists, and the Double Cross System [7]. Double Cross referred to turning all surviving German spies in the UK into double agents, who sent back convincing reports that were consistent with the deception programs being conducted by the London Controlling Section.
Wednesday, March 11, 2009
♣♦♣simple machines♣♦♣
Simple machine
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Table of simple mechanisms, from 1728 in "Cyclopaedia"[1]. Simple machines provide a "vocabulary" for understanding more complex machines.
This article is about the concept in physics. For the Internet forum software, see Simple Machines Forum.
A simple machine is a mechanical device that changes the direction or magnitude of a force.[2] In general, they can be defined as the simplest mechanisms that use mechanical advantage (also called leverage) to multiply force.[3] A simple machine uses a single applied force to do work against a single load force. Ignoring friction losses, the work done on the load is equal to the work done by the applied force. They can be used to increase the amount of the output force, at the cost of a proportional decrease in the distance moved by the load. The ratio of the output to the input force is called the mechanical advantage.
Usually the term refers to the six classical simple machines which were defined by Renaissance scientists:[4]
* Lever
* Wheel and axle
* Pulley
* Inclined plane
* Wedge
* Screw
They are the elementary "building blocks" of which all complicated machines are composed.[3][5] For example, wheels, levers, and pulleys are all used in the mechanism of a bicycle.
Simple machines fall into two classes; those dependent on the vector resolution of forces (inclined plane, wedge, screw) and those in which there is an equilibrium of torques (lever, pulley, wheel).
Contents
[hide]
* 1 History
* 2 Alternate definitions
* 3 Frictionless analysis
* 4 Footnotes
[edit] History
The idea of a "simple machine" originated with the Greek philosopher Archimedes around the 3rd century BC, who studied the "Archimedean" simple machines: lever, pulley, and screw. He discovered the principle of mechanical advantage in the lever.[6] His understanding was limited to the static balance of forces and did not include the tradeoff between force and distance moved. Heron of Alexandria (ca. 10–75 AD) in his work Mechanics lists five mechanisms with which a load can be set in motion: winch, lever, pulley, wedge, and screw.[7] During the Renaissance the classic five simple machines (excluding the wedge) began to be studied as a group. The complete dynamic theory of simple machines was worked out by Italian scientist Galileo Galilei in 1600 in Le Meccaniche ("On Mechanics"). He was the first to understand that simple machines do not create energy, only transform it.[8]
[edit] Alternate definitions
Any list of simple machines is somewhat arbitrary; the central idea is that every mechanism that manipulates force should be able to be understood as a combination of devices on the list. Some variations that have been proposed to the classical list of six simple machines:
* Some say there are only five simple machines, arguing that the wedge is a moving inclined plane.
* Others further simplify the list to four saying that the screw is a helical inclined plane.[9] This position is less accepted because a screw simultaneously converts a rotational force (torque) to a linear force.
* Some go even further to insist that only two simple machines exist, as a pulley and wheel and axle can be viewed as unique forms of levers, leaving only the lever and the inclined plane.[10][11][12][13]
* Hydraulic systems can also provide amplification of force, so some say they should be added to the list.[14][15][12]
[edit] Frictionless analysis
Although each machine works differently, the way they function is similar mathematically. In each machine, a force F_{in}\, is applied to the device at one point, and it does work moving a load, F_{out}\, at another point. Although some machines only change the direction of the force, such as a stationary pulley, most machines multiply (or divide) the magnitude of the force by a factor, the mechanical advantage, that can be calculated from the machine's geometry. For example, the mechanical advantage of a lever is equal to the ratio of its lever arms.
Simple machines do not contain a source of energy, so they cannot do more work than they receive from the input force. When friction and elasticity are ignored, the work output (that is done on the load) is equal to the work input (from the applied force). The work is defined as the force multiplied by the distance it moves. So the applied force, times the distance the input point moves, D_{in}\,, must be equal to the load force, times the distance the load moves, D_{out}\,[13]:
F_{in}D_{in} = F_{out}D_{out}.\,
So the ratio of output to input force, the mechanical advantage, is the inverse ratio of distances moved:
Mechanical Advantage \equiv \frac{F_{out}}{F_{in}} = \frac{D_{in}}{D_{out}}. \,
In the screw, which uses rotational motion, the input force should be replaced by the torque, and the distance by the angle the shaft is turned.
[edit] Footnotes
1. ^ Table of Mechanicks, from Ephraim Chambers (1728) Cyclopaedia, A Useful Dictionary of Arts and Sciences, Vol. 2, London, p.528, Plate 11.
2. ^ Paul, Akshoy; Pijush Roy, Sanchayan Mukherjee (2005). Mechanical Sciences:Engineering Mechanics and Strength of Materials. Prentice Hall of India. p. 215. ISBN 8120326113. http://www.mtsu.edu/~pdlee/public2_html/simple_machines.html#sm#5.
3. ^ a b Asimov, Isaac (1988). Understanding Physics. New York: Barnes & Noble. p. 88. ISBN 0880292512. http://books.google.com/books?id=pSKvaLV6zkcC&vq=archimedes&source=gbs_summary_s&cad=0.
4. ^ Anderson, William Ballantyne (1914). Physics for Technical Students: Mechanics and Heat. New York, USA: McGraw Hill. p. 112–122. http://books.google.com/books?id=Pa0IAAAAIAAJ&pg=PA112. Retrieved on 2008-05-11.
5. ^ Wallenstein, Andrew (June 2002). "Foundations of cognitive support: Toward abstract patterns of usefulness". Proceedings of the 9th Annual Workshop on the Design, Specification, and Verification of Interactive Systems, Springer. Retrieved on 2008-05-21.
6. ^ Ostdiek, Vern; Bord, Donald (2005). Inquiry into Physics. Thompson Brooks/Cole. p. 123. ISBN 0534491685. http://books.google.com/books?id=7kz2pd14hPUC&pg=PA123&sig=zOszHawWGqjbLs39NT9h_RidUGI. Retrieved on 2008-05-21.
7. ^ Strizhak, Viktor; Igor Penkov, Toivo Pappel (2004). "Evolution of design, use, and strength calculations of screw threads and threaded joints". HMM2004 International Symposium on History of Machines and Mechanisms, Kluwer Academic publishers. ISBN 1402022034. Retrieved on 2008-05-21.
8. ^ Krebs, Robert E. (2004). Groundbreaking Experiments, Inventions, and Discoveries of the Middle Ages. Greenwood Publishing Group. p. 163. ISBN 0313324336. http://books.google.com/books?id=MTXdplfiz-cC&dq=%22simple+machines%22+vector&lr=&as_brr=3&source=gbs_summary_s&cad=0. Retrieved on 2008-05-21.
9. ^ Carhart, Henry S.; Chute, Horatio N. (1917). Physics with Applications. Allyn & Bacom. pp. 159–160. http://books.google.com/books?id=4T0AAAAAYAAJ&pg=RA1-PA160. Retrieved on 2008-05-20.
10. ^ Isbell, Pam (2001). "Simple machines, or are they?". Grade 5–7 lesson plan. 2001 National Teacher Training Institute. http://www.myetv.org/education/ntti/lessons/2001_lessons/simplemachines.cfm. Retrieved on 2008-05-13.
11. ^ Clute, Willard N. (1917). Experimental General Science. Philadelphia: P. Blakiston's Son & Co.. pp. 188. http://books.google.com/books?id=OuFHAAAAIAAJ&pg=PA188. Retrieved on 2008-05-20.
12. ^ a b "Mechanical Advantage and Simple Machines". BNET Business Network. CNET. 2002. http://findarticles.com/p/articles/mi_gx5226/is_2002/ai_n19143765/pg_1. Retrieved on 2008-05-21.
13. ^ a b Beiser, Arthur (2004). Schaum's Outline of Applied Physics. McGraw-Hill. p. 145. ISBN 0071426116. http://books.google.com/books?id=soKguvJDgmsC&dq=Hydraulic+%22simple+machines%22&client=opera&source=gbs_summary_s&cad=0. Retrieved on 2008-05-21.
14. ^ This was first suggested by Blaise Pascal in the 17th century: Meli, Domenico Bertolini (2006). Thinking with Objects:The Transformation of Mechanics in the 17th Century. JHU Press. ISBN 0801884276. http://books.google.com/books?id=qbS_0qAB3_cC&dq=Hydraulic+%22simple+machines%22&client=opera&source=gbs_summary_s&cad=0. p.175
15. ^ "Mechanical Advantage - Simple Machines". MCAT Exam preparation. Eduturca. January 7, 2008. http://www.eduturca.com/mcat-exam/mechanical-advantage-simple-machines-mcat.html. Retrieved on 2008-05-21.
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Table of simple mechanisms, from 1728 in "Cyclopaedia"[1]. Simple machines provide a "vocabulary" for understanding more complex machines.
This article is about the concept in physics. For the Internet forum software, see Simple Machines Forum.
A simple machine is a mechanical device that changes the direction or magnitude of a force.[2] In general, they can be defined as the simplest mechanisms that use mechanical advantage (also called leverage) to multiply force.[3] A simple machine uses a single applied force to do work against a single load force. Ignoring friction losses, the work done on the load is equal to the work done by the applied force. They can be used to increase the amount of the output force, at the cost of a proportional decrease in the distance moved by the load. The ratio of the output to the input force is called the mechanical advantage.
Usually the term refers to the six classical simple machines which were defined by Renaissance scientists:[4]
* Lever
* Wheel and axle
* Pulley
* Inclined plane
* Wedge
* Screw
They are the elementary "building blocks" of which all complicated machines are composed.[3][5] For example, wheels, levers, and pulleys are all used in the mechanism of a bicycle.
Simple machines fall into two classes; those dependent on the vector resolution of forces (inclined plane, wedge, screw) and those in which there is an equilibrium of torques (lever, pulley, wheel).
Contents
[hide]
* 1 History
* 2 Alternate definitions
* 3 Frictionless analysis
* 4 Footnotes
[edit] History
The idea of a "simple machine" originated with the Greek philosopher Archimedes around the 3rd century BC, who studied the "Archimedean" simple machines: lever, pulley, and screw. He discovered the principle of mechanical advantage in the lever.[6] His understanding was limited to the static balance of forces and did not include the tradeoff between force and distance moved. Heron of Alexandria (ca. 10–75 AD) in his work Mechanics lists five mechanisms with which a load can be set in motion: winch, lever, pulley, wedge, and screw.[7] During the Renaissance the classic five simple machines (excluding the wedge) began to be studied as a group. The complete dynamic theory of simple machines was worked out by Italian scientist Galileo Galilei in 1600 in Le Meccaniche ("On Mechanics"). He was the first to understand that simple machines do not create energy, only transform it.[8]
[edit] Alternate definitions
Any list of simple machines is somewhat arbitrary; the central idea is that every mechanism that manipulates force should be able to be understood as a combination of devices on the list. Some variations that have been proposed to the classical list of six simple machines:
* Some say there are only five simple machines, arguing that the wedge is a moving inclined plane.
* Others further simplify the list to four saying that the screw is a helical inclined plane.[9] This position is less accepted because a screw simultaneously converts a rotational force (torque) to a linear force.
* Some go even further to insist that only two simple machines exist, as a pulley and wheel and axle can be viewed as unique forms of levers, leaving only the lever and the inclined plane.[10][11][12][13]
* Hydraulic systems can also provide amplification of force, so some say they should be added to the list.[14][15][12]
[edit] Frictionless analysis
Although each machine works differently, the way they function is similar mathematically. In each machine, a force F_{in}\, is applied to the device at one point, and it does work moving a load, F_{out}\, at another point. Although some machines only change the direction of the force, such as a stationary pulley, most machines multiply (or divide) the magnitude of the force by a factor, the mechanical advantage, that can be calculated from the machine's geometry. For example, the mechanical advantage of a lever is equal to the ratio of its lever arms.
Simple machines do not contain a source of energy, so they cannot do more work than they receive from the input force. When friction and elasticity are ignored, the work output (that is done on the load) is equal to the work input (from the applied force). The work is defined as the force multiplied by the distance it moves. So the applied force, times the distance the input point moves, D_{in}\,, must be equal to the load force, times the distance the load moves, D_{out}\,[13]:
F_{in}D_{in} = F_{out}D_{out}.\,
So the ratio of output to input force, the mechanical advantage, is the inverse ratio of distances moved:
Mechanical Advantage \equiv \frac{F_{out}}{F_{in}} = \frac{D_{in}}{D_{out}}. \,
In the screw, which uses rotational motion, the input force should be replaced by the torque, and the distance by the angle the shaft is turned.
[edit] Footnotes
1. ^ Table of Mechanicks, from Ephraim Chambers (1728) Cyclopaedia, A Useful Dictionary of Arts and Sciences, Vol. 2, London, p.528, Plate 11.
2. ^ Paul, Akshoy; Pijush Roy, Sanchayan Mukherjee (2005). Mechanical Sciences:Engineering Mechanics and Strength of Materials. Prentice Hall of India. p. 215. ISBN 8120326113. http://www.mtsu.edu/~pdlee/public2_html/simple_machines.html#sm#5.
3. ^ a b Asimov, Isaac (1988). Understanding Physics. New York: Barnes & Noble. p. 88. ISBN 0880292512. http://books.google.com/books?id=pSKvaLV6zkcC&vq=archimedes&source=gbs_summary_s&cad=0.
4. ^ Anderson, William Ballantyne (1914). Physics for Technical Students: Mechanics and Heat. New York, USA: McGraw Hill. p. 112–122. http://books.google.com/books?id=Pa0IAAAAIAAJ&pg=PA112. Retrieved on 2008-05-11.
5. ^ Wallenstein, Andrew (June 2002). "Foundations of cognitive support: Toward abstract patterns of usefulness". Proceedings of the 9th Annual Workshop on the Design, Specification, and Verification of Interactive Systems, Springer. Retrieved on 2008-05-21.
6. ^ Ostdiek, Vern; Bord, Donald (2005). Inquiry into Physics. Thompson Brooks/Cole. p. 123. ISBN 0534491685. http://books.google.com/books?id=7kz2pd14hPUC&pg=PA123&sig=zOszHawWGqjbLs39NT9h_RidUGI. Retrieved on 2008-05-21.
7. ^ Strizhak, Viktor; Igor Penkov, Toivo Pappel (2004). "Evolution of design, use, and strength calculations of screw threads and threaded joints". HMM2004 International Symposium on History of Machines and Mechanisms, Kluwer Academic publishers. ISBN 1402022034. Retrieved on 2008-05-21.
8. ^ Krebs, Robert E. (2004). Groundbreaking Experiments, Inventions, and Discoveries of the Middle Ages. Greenwood Publishing Group. p. 163. ISBN 0313324336. http://books.google.com/books?id=MTXdplfiz-cC&dq=%22simple+machines%22+vector&lr=&as_brr=3&source=gbs_summary_s&cad=0. Retrieved on 2008-05-21.
9. ^ Carhart, Henry S.; Chute, Horatio N. (1917). Physics with Applications. Allyn & Bacom. pp. 159–160. http://books.google.com/books?id=4T0AAAAAYAAJ&pg=RA1-PA160. Retrieved on 2008-05-20.
10. ^ Isbell, Pam (2001). "Simple machines, or are they?". Grade 5–7 lesson plan. 2001 National Teacher Training Institute. http://www.myetv.org/education/ntti/lessons/2001_lessons/simplemachines.cfm. Retrieved on 2008-05-13.
11. ^ Clute, Willard N. (1917). Experimental General Science. Philadelphia: P. Blakiston's Son & Co.. pp. 188. http://books.google.com/books?id=OuFHAAAAIAAJ&pg=PA188. Retrieved on 2008-05-20.
12. ^ a b "Mechanical Advantage and Simple Machines". BNET Business Network. CNET. 2002. http://findarticles.com/p/articles/mi_gx5226/is_2002/ai_n19143765/pg_1. Retrieved on 2008-05-21.
13. ^ a b Beiser, Arthur (2004). Schaum's Outline of Applied Physics. McGraw-Hill. p. 145. ISBN 0071426116. http://books.google.com/books?id=soKguvJDgmsC&dq=Hydraulic+%22simple+machines%22&client=opera&source=gbs_summary_s&cad=0. Retrieved on 2008-05-21.
14. ^ This was first suggested by Blaise Pascal in the 17th century: Meli, Domenico Bertolini (2006). Thinking with Objects:The Transformation of Mechanics in the 17th Century. JHU Press. ISBN 0801884276. http://books.google.com/books?id=qbS_0qAB3_cC&dq=Hydraulic+%22simple+machines%22&client=opera&source=gbs_summary_s&cad=0. p.175
15. ^ "Mechanical Advantage - Simple Machines". MCAT Exam preparation. Eduturca. January 7, 2008. http://www.eduturca.com/mcat-exam/mechanical-advantage-simple-machines-mcat.html. Retrieved on 2008-05-21.
A compound machine consists of two or more simple machines put together. In fact, most machines are compound machines. Compound machines can do more difficult jobs than simple machines alone. Their mechanical advantage is far greater, too. Some examples are a pair of scissors and a bicycle.
Click on the scissors to name the simple machines that make it up.
Click on the bicycle to name the simple machines that make it up.
Click on the scissors to name the simple machines that make it up.
Click on the bicycle to name the simple machines that make it up.
♦•♦joule♦•♦
Joule
From Wikipedia, the free encyclopedia
Jump to: navigation, search
For the physicist named James Prescott Joule, see James Prescott Joule.
For the electric car, see Optimal Energy Joule.
The joule is the derived unit of energy in the International System of Units. It is defined as:
\, 1\, \mathrm{J}=1\, \mathrm{kg} \cdot \frac{\mathrm{m}^{2}}{\mathrm{s}^{2}}
One joule is the amount of energy required to perform the following actions:
* The work done by a force of one newton traveling through a distance of one meter;
* The work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt, with the symbol C·V;
* The work done to produce the power of one watt continuously for one second; or one watt second (compare kilowatt hour), with the symbol W·s. Thus a kilowatt hour is 3,600,000 joules or 3.6 megajoules;
* The kinetic energy of a 2 kilogram (kg) mass (m) moving at a velocity of 1 meter per second (m/s). The energy is linear in the mass but quadratic in the velocity, being given by E = ½mv², energy (E) is equal to 1/2 of mass (m) multiplied by velocity (v) squared.
Contents
[hide]
* 1 Conversions
o 1.1 Practical examples
o 1.2 SI multiples
* 2 See also
* 3 References
* 4 External links
[edit] Conversions
Main article: Conversion of units#Energy
1 joule is exactly 107 ergs.
1 joule is exactly equal to:
* 6.2415 ×1018 eV (electronvolts)
* 0.2390 cal (calorie) (small calories, lower case c)
* 2.3901 ×10−4 kilocalorie, Calories (food energy, upper case C)
* 9.4782 ×10−4 BTU (British thermal unit)
* 0.7376 ft·lbf (foot-pound force)
* 23.7 ft·pdl (foot poundals)
* 2.7778 ×10−7 kilowatt hour
* 2.7778 ×10−4 watt hour
* 9.8692 ×10−3 litre-atmosphere
Units defined in terms of the joule include:
* 1 thermochemical calorie = 4.184 J
* 1 International Table calorie = 4.1868 J
* 1 watt hour = 3600 J
* 1 kilowatt hour = 3.6 ×106 J (or 3.6 MJ)
* 1 ton TNT exploding = 4.184 GJ
Useful to remember:
* 1 joule = 1 newton meter = 1 watt second
[edit] Practical examples
One joule in everyday life is approximately:
* the energy required to lift a small apple one meter straight up.
* the energy released when that same apple falls one meter to the ground.
* the energy released as heat by a quiet person, every hundredth of a second.
* the energy required to heat one gram of dry, cool air by 1 degree Celsius.
* one hundredth of the energy a person can receive by drinking a drop of beer.
* the kinetic energy of an adult human moving a distance of about a handspan every second.
[edit] SI multiples
SI multiples for joule (J) Submultiples Multiples
Value Symbol Name Value Symbol Name
10–1 J dJ decijoule 101 J daJ decajoule
10–2 J cJ centijoule 102 J hJ hectojoule
10–3 J mJ millijoule 103 J kJ kilojoule
10–6 J µJ microjoule 106 J MJ megajoule
10–9 J nJ nanojoule 109 J GJ gigajoule
10–12 J pJ picojoule 1012 J TJ terajoule
10–15 J fJ femtojoule 1015 J PJ petajoule
10–18 J aJ attojoule 1018 J EJ exajoule
10–21 J zJ zeptojoule 1021 J ZJ zettajoule
10–24 J yJ yoctojoule 1024 J YJ yottajoule
Common multiples are in bold face
This SI unit is named after James Prescott Joule. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (J). When an SI unit is spelled out in English, it should always begin with a lowercase letter (joule), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.
—Based on The International System of Units, section 5.2.
From Wikipedia, the free encyclopedia
Jump to: navigation, search
For the physicist named James Prescott Joule, see James Prescott Joule.
For the electric car, see Optimal Energy Joule.
The joule is the derived unit of energy in the International System of Units. It is defined as:
\, 1\, \mathrm{J}=1\, \mathrm{kg} \cdot \frac{\mathrm{m}^{2}}{\mathrm{s}^{2}}
One joule is the amount of energy required to perform the following actions:
* The work done by a force of one newton traveling through a distance of one meter;
* The work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt, with the symbol C·V;
* The work done to produce the power of one watt continuously for one second; or one watt second (compare kilowatt hour), with the symbol W·s. Thus a kilowatt hour is 3,600,000 joules or 3.6 megajoules;
* The kinetic energy of a 2 kilogram (kg) mass (m) moving at a velocity of 1 meter per second (m/s). The energy is linear in the mass but quadratic in the velocity, being given by E = ½mv², energy (E) is equal to 1/2 of mass (m) multiplied by velocity (v) squared.
Contents
[hide]
* 1 Conversions
o 1.1 Practical examples
o 1.2 SI multiples
* 2 See also
* 3 References
* 4 External links
[edit] Conversions
Main article: Conversion of units#Energy
1 joule is exactly 107 ergs.
1 joule is exactly equal to:
* 6.2415 ×1018 eV (electronvolts)
* 0.2390 cal (calorie) (small calories, lower case c)
* 2.3901 ×10−4 kilocalorie, Calories (food energy, upper case C)
* 9.4782 ×10−4 BTU (British thermal unit)
* 0.7376 ft·lbf (foot-pound force)
* 23.7 ft·pdl (foot poundals)
* 2.7778 ×10−7 kilowatt hour
* 2.7778 ×10−4 watt hour
* 9.8692 ×10−3 litre-atmosphere
Units defined in terms of the joule include:
* 1 thermochemical calorie = 4.184 J
* 1 International Table calorie = 4.1868 J
* 1 watt hour = 3600 J
* 1 kilowatt hour = 3.6 ×106 J (or 3.6 MJ)
* 1 ton TNT exploding = 4.184 GJ
Useful to remember:
* 1 joule = 1 newton meter = 1 watt second
[edit] Practical examples
One joule in everyday life is approximately:
* the energy required to lift a small apple one meter straight up.
* the energy released when that same apple falls one meter to the ground.
* the energy released as heat by a quiet person, every hundredth of a second.
* the energy required to heat one gram of dry, cool air by 1 degree Celsius.
* one hundredth of the energy a person can receive by drinking a drop of beer.
* the kinetic energy of an adult human moving a distance of about a handspan every second.
[edit] SI multiples
SI multiples for joule (J) Submultiples Multiples
Value Symbol Name Value Symbol Name
10–1 J dJ decijoule 101 J daJ decajoule
10–2 J cJ centijoule 102 J hJ hectojoule
10–3 J mJ millijoule 103 J kJ kilojoule
10–6 J µJ microjoule 106 J MJ megajoule
10–9 J nJ nanojoule 109 J GJ gigajoule
10–12 J pJ picojoule 1012 J TJ terajoule
10–15 J fJ femtojoule 1015 J PJ petajoule
10–18 J aJ attojoule 1018 J EJ exajoule
10–21 J zJ zeptojoule 1021 J ZJ zettajoule
10–24 J yJ yoctojoule 1024 J YJ yottajoule
Common multiples are in bold face
This SI unit is named after James Prescott Joule. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (J). When an SI unit is spelled out in English, it should always begin with a lowercase letter (joule), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.
—Based on The International System of Units, section 5.2.
•♦•neWt0n meTre•♦•
Newton metre
From Wikipedia, the free encyclopedia
(Redirected from Newton meter)
Jump to: navigation, search
Newton metre is a unit of torque (also called "moment") in the SI system.[1] The symbolic form is N m or N·m,[2] and sometimes hyphenated newton-metre. One newton metre is equal to the torque resulting from a force of one newton applied perpendicularly to a moment arm which is one metre long.
A newton metre is dimensionally equal to a joule, the SI unit of energy and work. However, it is not appropriate to express a torque in joules — the units are necessary to distinguish a torque quantity from an energy quantity.[3] The two quantities, torque and energy, are physically different despite being dimensionally equivalent. For example, energy is a scalar while torque is a vector (in fact, a pseudovector). Also, a given force applied to an object can contribute torque but not work, or work but not torque, or both, or neither.
On the other hand, there are relations between torque and energy that sheds light on their dimensional relationship. In particular, a torque can contribute to rotational energy; the work done in this process (measured in J) is equal to the torque (measured in N m) times the angle through which the body rotates in the direction of the torque.[4] This accounts for the use of an alternative unit for torque, Joule per radian (J/rad)
[edit] Conversion factors
* 1 joule = 1 N·m
* 1 newton metre = 0.7375621 foot-pound force (often "foot-pound")
* 1 metre kilogram-force = 9.80665 N·m
* 1 centimetre kilogram-force = 98.0665 mN·m
* 1 foot-pound force (often "foot-pounds") = 1 pound-force foot (often "pound-feet") ≈ 1.3558 N·m
* 1 inch ounce-force = 7.0615518 mN·m
* 1 dyne centimetre = 10−7 N·m
From Wikipedia, the free encyclopedia
(Redirected from Newton meter)
Jump to: navigation, search
Newton metre is a unit of torque (also called "moment") in the SI system.[1] The symbolic form is N m or N·m,[2] and sometimes hyphenated newton-metre. One newton metre is equal to the torque resulting from a force of one newton applied perpendicularly to a moment arm which is one metre long.
A newton metre is dimensionally equal to a joule, the SI unit of energy and work. However, it is not appropriate to express a torque in joules — the units are necessary to distinguish a torque quantity from an energy quantity.[3] The two quantities, torque and energy, are physically different despite being dimensionally equivalent. For example, energy is a scalar while torque is a vector (in fact, a pseudovector). Also, a given force applied to an object can contribute torque but not work, or work but not torque, or both, or neither.
On the other hand, there are relations between torque and energy that sheds light on their dimensional relationship. In particular, a torque can contribute to rotational energy; the work done in this process (measured in J) is equal to the torque (measured in N m) times the angle through which the body rotates in the direction of the torque.[4] This accounts for the use of an alternative unit for torque, Joule per radian (J/rad)
[edit] Conversion factors
* 1 joule = 1 N·m
* 1 newton metre = 0.7375621 foot-pound force (often "foot-pound")
* 1 metre kilogram-force = 9.80665 N·m
* 1 centimetre kilogram-force = 98.0665 mN·m
* 1 foot-pound force (often "foot-pounds") = 1 pound-force foot (often "pound-feet") ≈ 1.3558 N·m
* 1 inch ounce-force = 7.0615518 mN·m
* 1 dyne centimetre = 10−7 N·m
•◘•baR0meTer•◘•
Barometer
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Schematic drawing of a simple mercury barometer with vertical mercury column and reservoir at base
Goethe's device
A barometer is an instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or mercury. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems, and frontal boundaries.
Contents
[hide]
* 1 History
* 2 Types
o 2.1 Water-based barometers
o 2.2 Mercury barometers
o 2.3 Aneroid barometers
o 2.4 Barographs
* 3 Applications
* 4 Compensations
o 4.1 Temperature
o 4.2 Altitude
* 5 Patents
* 6 References
* 7 Further reading
* 8 See also
* 9 External links
[edit] History
Although Evangelista Torricelli[1][2][3] is universally credited with inventing the barometer in 1643, two other noteworthy efforts must be cited. Historical documentation also suggests Gasparo Berti, an Italian mathematician and astronomer, built unintentionally water barometer sometime between 1640 and 1643.[1][4] French scientist and philosopher Rene Descartes described the design of an experiment on atmospheric pressure determination as early as 1631, but there is no evidence that he built a working barometer at that time.[1]
[edit] Types
[edit] Water-based barometers
This concept of "decreasing atmospheric pressure predicts stormy weather" was postulated by Lucien Vidie and is the basis for a basic weather prediction device called a weather glass or thunder glass. It can also be called a "storm glass" or a "Goethe barometer" (the writer Goethe popularized it in Germany). It consists of a glass container with a sealed body, half filled with water. A narrow spout connects to the body below the water level and rises above the water level, where it is open to the atmosphere. When the air pressure is lower than it was at the time the body was sealed, the water level in the spout will rise above the water level in the body; when the air pressure is higher, the water level in the spout will drop below the water level in the body. A variation of this type of barometer can be easily made at home.[5]
[edit] Mercury barometers
A mercury barometer has a glass tube of about 30 inches (about 76 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. Since higher temperature at the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect.
Torricelli documented that the height of the mercury in a barometer changed slightly each day and concluded that this was due to the changing pressure in the atmosphere[6]. He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".
The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters (torr): the pressure is quoted as the level of the mercury's height in the vertical column. 1 atmosphere is equivalent to about 29.9 inches, or 760 millimeters, of mercury. The use of this unit is still popular in the United States, although it has been disused in favor of SI or metric units in other parts of the world. Barometers of this type normally measure atmospheric pressures between 28 and 31 inches of mercury.
Design changes to make the instrument more sensitive, simpler to read, and easier to transport resulted in variations such as the basin, siphon, wheel, cistern, Fortin, multiple folded, stereometric, and balance barometers. Fitzroy barometers combine the standard mercury barometer with a thermometer, as well as a guide of how to interpret pressure changes.
On June 5, 2007, a European Union directive was enacted to restrict the sale of mercury, thus effectively ending the production of new mercury barometers in Europe.
[edit] Aneroid barometers
See also: Barograph
Old aneroid barometer
Modern aneroid barometer
An aneroid barometer uses a small, flexible metal box called an aneroid cell. This aneroid capsule (cell) is made from an alloy of beryllium and copper.[7] The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen. In addition, the mechanism is made deliberately 'stiff' so that tapping the barometer reveals whether the pressure is rising or falling as the pointer moves.
[edit] Barographs
A barograph, which records a graph of some atmospheric pressure, uses an aneroid barometer mechanism to move a needle on a smoked foil or to move a pen upon paper, both of which are attached to a drum moved by clockwork.[8] Barographs may be calibrated for altitude and this type is often used to preserve a record of balloon and glider flights.
[edit] Applications
See also: Surface weather analysis
See also: Weather forecasting
Barograph using five stacked aneroid barometer cells.
A barometer is commonly used for weather prediction, as high air pressure in a region indicates fair weather while low pressure indicates that storms are more likely. When used in combination with wind observations, reasonably accurate short term forecasts can be made.[9] Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced, which were the first form of the modern weather map when created in the 19th century. Isobars, lines of equal pressure, when drawn on such a map, gives a contour map showing areas of high and low pressure. Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Low atmospheric pressure, on the other hand, represents the path of least resistance for a weather system, making it more likely that low pressure will be associated with increased storm activities. If the barometer is falling then deteriorating weather or some form of precipitation will fall, however if the barometer is rising then there will be nice weather or no precipitation.
[edit] Compensations
[edit] Temperature
The density of mercury will change with temperature, so a reading must be adjusted for the temperature of the instrument. For this purpose a mercury thermometer is usually mounted on the instrument. Temperature compensation of an aneroid barometer is accomplished by including a bi-metal element in the mechanical linkages. Aneroid barometers sold for domestic use seldom go to the trouble.
[edit] Altitude
As the air pressure will be decreased at altitudes above sea level (and increased below sea level) the actual reading of the instrument will be dependent upon its location. This pressure is then converted to an equivalent sea-level pressure for purposes of reporting and for adjusting aircraft altimeters (as aircraft may fly between regions of varying normalized atmospheric pressure owing to the presence of weather systems). Aneroid barometers have a mechanical adjustment for altitude that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude.
[edit] Patents
Table of Pneumaticks, 1728 Cyclopaedia
* US patent 2194624, "Diaphragm pressure gauge having temperature compensating means", granted 1940-03-26, assigned to Bendix Aviat Corp
* U.S. Patent 2,472,735 : C. J. Ulrich : "Barometric instrument"
* U.S. Patent 2,691,305 : H. J. Frank : Barometric altimeter"
* U.S. Patent 3,273,398 : D. C. W. T. Sharp : "Aneroid barometer"
* U.S. Patent 3,397,578 : H. A. Klumb : "Motion amplifying mechanism for pressure responsive instrument movement"
* U.S. Patent 3,643,510 : F. Lissau : "Fluid displacement pressure gauges"
* U.S. Patent 4,106,342 : O. S. Sormunen : "Pressure measuring instrument"
* U.S. Patent 4,238,958 : H. Dostmann : "Barometer"
* U.S. Patent 4,327,583 : T. Fijimoto : "Weather forecasting device"
From Wikipedia, the free encyclopedia
Jump to: navigation, search
Schematic drawing of a simple mercury barometer with vertical mercury column and reservoir at base
Goethe's device
A barometer is an instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or mercury. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems, and frontal boundaries.
Contents
[hide]
* 1 History
* 2 Types
o 2.1 Water-based barometers
o 2.2 Mercury barometers
o 2.3 Aneroid barometers
o 2.4 Barographs
* 3 Applications
* 4 Compensations
o 4.1 Temperature
o 4.2 Altitude
* 5 Patents
* 6 References
* 7 Further reading
* 8 See also
* 9 External links
[edit] History
Although Evangelista Torricelli[1][2][3] is universally credited with inventing the barometer in 1643, two other noteworthy efforts must be cited. Historical documentation also suggests Gasparo Berti, an Italian mathematician and astronomer, built unintentionally water barometer sometime between 1640 and 1643.[1][4] French scientist and philosopher Rene Descartes described the design of an experiment on atmospheric pressure determination as early as 1631, but there is no evidence that he built a working barometer at that time.[1]
[edit] Types
[edit] Water-based barometers
This concept of "decreasing atmospheric pressure predicts stormy weather" was postulated by Lucien Vidie and is the basis for a basic weather prediction device called a weather glass or thunder glass. It can also be called a "storm glass" or a "Goethe barometer" (the writer Goethe popularized it in Germany). It consists of a glass container with a sealed body, half filled with water. A narrow spout connects to the body below the water level and rises above the water level, where it is open to the atmosphere. When the air pressure is lower than it was at the time the body was sealed, the water level in the spout will rise above the water level in the body; when the air pressure is higher, the water level in the spout will drop below the water level in the body. A variation of this type of barometer can be easily made at home.[5]
[edit] Mercury barometers
A mercury barometer has a glass tube of about 30 inches (about 76 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. Since higher temperature at the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect.
Torricelli documented that the height of the mercury in a barometer changed slightly each day and concluded that this was due to the changing pressure in the atmosphere[6]. He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".
The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters (torr): the pressure is quoted as the level of the mercury's height in the vertical column. 1 atmosphere is equivalent to about 29.9 inches, or 760 millimeters, of mercury. The use of this unit is still popular in the United States, although it has been disused in favor of SI or metric units in other parts of the world. Barometers of this type normally measure atmospheric pressures between 28 and 31 inches of mercury.
Design changes to make the instrument more sensitive, simpler to read, and easier to transport resulted in variations such as the basin, siphon, wheel, cistern, Fortin, multiple folded, stereometric, and balance barometers. Fitzroy barometers combine the standard mercury barometer with a thermometer, as well as a guide of how to interpret pressure changes.
On June 5, 2007, a European Union directive was enacted to restrict the sale of mercury, thus effectively ending the production of new mercury barometers in Europe.
[edit] Aneroid barometers
See also: Barograph
Old aneroid barometer
Modern aneroid barometer
An aneroid barometer uses a small, flexible metal box called an aneroid cell. This aneroid capsule (cell) is made from an alloy of beryllium and copper.[7] The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen. In addition, the mechanism is made deliberately 'stiff' so that tapping the barometer reveals whether the pressure is rising or falling as the pointer moves.
[edit] Barographs
A barograph, which records a graph of some atmospheric pressure, uses an aneroid barometer mechanism to move a needle on a smoked foil or to move a pen upon paper, both of which are attached to a drum moved by clockwork.[8] Barographs may be calibrated for altitude and this type is often used to preserve a record of balloon and glider flights.
[edit] Applications
See also: Surface weather analysis
See also: Weather forecasting
Barograph using five stacked aneroid barometer cells.
A barometer is commonly used for weather prediction, as high air pressure in a region indicates fair weather while low pressure indicates that storms are more likely. When used in combination with wind observations, reasonably accurate short term forecasts can be made.[9] Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced, which were the first form of the modern weather map when created in the 19th century. Isobars, lines of equal pressure, when drawn on such a map, gives a contour map showing areas of high and low pressure. Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Low atmospheric pressure, on the other hand, represents the path of least resistance for a weather system, making it more likely that low pressure will be associated with increased storm activities. If the barometer is falling then deteriorating weather or some form of precipitation will fall, however if the barometer is rising then there will be nice weather or no precipitation.
[edit] Compensations
[edit] Temperature
The density of mercury will change with temperature, so a reading must be adjusted for the temperature of the instrument. For this purpose a mercury thermometer is usually mounted on the instrument. Temperature compensation of an aneroid barometer is accomplished by including a bi-metal element in the mechanical linkages. Aneroid barometers sold for domestic use seldom go to the trouble.
[edit] Altitude
As the air pressure will be decreased at altitudes above sea level (and increased below sea level) the actual reading of the instrument will be dependent upon its location. This pressure is then converted to an equivalent sea-level pressure for purposes of reporting and for adjusting aircraft altimeters (as aircraft may fly between regions of varying normalized atmospheric pressure owing to the presence of weather systems). Aneroid barometers have a mechanical adjustment for altitude that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude.
[edit] Patents
Table of Pneumaticks, 1728 Cyclopaedia
* US patent 2194624, "Diaphragm pressure gauge having temperature compensating means", granted 1940-03-26, assigned to Bendix Aviat Corp
* U.S. Patent 2,472,735 : C. J. Ulrich : "Barometric instrument"
* U.S. Patent 2,691,305 : H. J. Frank : Barometric altimeter"
* U.S. Patent 3,273,398 : D. C. W. T. Sharp : "Aneroid barometer"
* U.S. Patent 3,397,578 : H. A. Klumb : "Motion amplifying mechanism for pressure responsive instrument movement"
* U.S. Patent 3,643,510 : F. Lissau : "Fluid displacement pressure gauges"
* U.S. Patent 4,106,342 : O. S. Sormunen : "Pressure measuring instrument"
* U.S. Patent 4,238,958 : H. Dostmann : "Barometer"
* U.S. Patent 4,327,583 : T. Fijimoto : "Weather forecasting device"
○◘○atn0Spher!c prEssuRe○◘○
Atmospheric pressure
From Wikipedia, the free encyclopedia
Jump to: navigation, search
"Air pressure" redirects here. For the pressure of air in other systems, see pressure.
15 qyear average MSLP for JJA (top) and DJF (bottom).
Atmospheric pressure is sometimes defined as the force per unit area exerted against a surface by the weight of air above that surface at any given point in the Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. A column of air one square inch in cross-section, measured from sea level to the top of the atmosphere, would weigh approximately 14.7 lbf. The weight of a 1 m2 (11 sq ft) column of air would be about 100 kilonewtons (equivalent to a mass of 10.2 tonnes at the surface).
Contents
[hide]
* 1 Standard atmospheric pressure
* 2 Mean sea level pressure
* 3 Altitude atmospheric pressure variation
* 4 Calculating variation with altitude
* 5 Local atmospheric pressure variation
* 6 Atmospheric pressure based on height of water
* 7 Water's boiling point
* 8 Effect on human health
* 9 See also
* 10 Notes
* 11 References
* 12 External links
o 12.1 Experiments
[edit] Standard atmospheric pressure
The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being equal to 101.325 kPa. These other units are equivalent: 760 mmHg (torr), 29.92 inHg, 14.696 PSI, 1013.25 millibars. One standard atmosphere is standard pressure used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries.
In 1999, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the properties of substances, “the standard pressure” should be defined as precisely 100 kPa (≈750.01 torr) or 29.53 inHg rather than the 101.325 kPa value of “one standard atmosphere”.[1] This value is used as the standard pressure for the compressor and the pneumatic tool industries (ISO 2787).[2] (See also Standard temperature and pressure.) In the United States, compressed air flow is often measured in "standard cubic feet" per unit of time, where the "standard" means the equivalent quantity of moisture at standard temperature and pressure. However, this standard atmosphere is defined slightly differently: temperature = 20 °C (68 °F), air density = 1.225 kg/m³ (0.0765 lb/cu ft), altitude = sea level, and relative humidity = 20%. In the air conditioning industry, the standard is often temperature = 0 °C (32 °F) instead. For natural gas, the petroleum industry uses a standard temperature of 15.6 °C (60.1 °F), pressure 101.56 kPa (14.730 psi). (air pressure)
[edit] Mean sea level pressure
Mean sea level pressure (MSLP or QFF) is the pressure at sea level or (when measured at a given elevation on land) the station pressure reduced to sea level assuming an isothermal layer at the station temperature.
This is the pressure normally given in weather reports on radio, television, and newspapers or on the Internet. When barometers in the home are set to match the local weather reports, they measure pressure reduced to sea level, not the actual local atmospheric pressure. See Altimeter (barometer vs. absolute).
The reduction to sea level means that the normal range of fluctuations in pressure is the same for everyone. The pressures which are considered high pressure or low pressure do not depend on geographical location. This makes isobars on a weather map meaningful and useful tools.
Kollsman-type barometric aircraft altimeter as used in North America displaying an altitude of 80 ft (24 m).
The altimeter setting in aviation, set either QNH or QFE, is another atmospheric pressure reduced to sea level, but the method of making this reduction differs slightly.
QNH
The barometric altimeter setting which will cause the altimeter to read airfield elevation when on the airfield. In ISA temperature conditions the altimeter will read altitude above mean sea level in the vicinity of the airfield
QFE
The barometric altimeter setting which will cause an altimeter to read zero when at the reference datum of a particular airfield (generally a runway threshold). In ISA temperature conditions the altimeter will read height above the datum in the vicinity of the airfield.
QFE and QNH are arbitrary Q codes rather than abbreviations, but the mnemonics "Nautical Height" (for QNH) and "Field Elevation" (for QFE) are often used by pilots to distinguish them.
Average sea-level pressure is 101.325 kPa (1013.25 mbar) or 29.921 inches of mercury (inHg) or 760 millimeters (mmHg). In aviation weather reports (METAR), QNH is transmitted around the world in millibars or hectopascals (1 millibar = 1 hectopascal), except in the United States and in Canada where it is reported in inches (or hundredths of inches) of mercury. (The United States and Canada also report sea level pressure SLP, which is reduced to sea level by a different method, in the remarks section, not an internationally transmitted part of the code, in hectopascals or millibars [3]. However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals [1], while Environment Canada's standard unit of pressure is the same [2] [3].) In the weather code, three digits are all that is needed; decimal points and the one or two most significant digits are omitted: 1013.2 mbar or 101.32 kPa is transmitted as 132; 1000.0 mbar or 100.00 kPa is transmitted as 000; 998.7 mbar or 99.87 kPa is transmitted as 987; etc. The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1087.0 mbar. The lowest measurable sea-level pressure is found at the centers of tropical cyclones.
[edit] Altitude atmospheric pressure variation
This plastic bottle was closed at approximately 2,000 m (6,600 ft) altitude, then brought back to sea level. It was crushed by air pressure.
Pressure varies smoothly from the earth's surface to the top of the mesosphere. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. The following is a list of air pressures (as a fraction of one atmosphere) with the corresponding average altitudes. The table gives a rough idea of air pressure at various altitudes.
fraction of 1 atm average altitude
(m) (ft)
1 0 0
1/2 5,486 18,000
1/3 8,376 27,480
1/10 16,132 52,926
1/100 30,901 101,381
1/1000 48,467 159,013
1/10000 69,464 227,899
1/100000 86,282 283,076
[edit] Calculating variation with altitude
See also: Barometric formula
There are two different equations for computing the average pressure at various height regimes below 86 km (53 mi; 280,000 ft). Equation 1 is used when the value of standard temperature lapse rate is not equal to zero and equation 2 is used when standard temperature lapse rate equals zero.
Equation 1:
{P}=P_b \cdot \left[\frac{T_b}{T_b + L_b\cdot(h-h_b)}\right]^{\textstyle \frac{g_0 \cdot M}{R^* \cdot L_b}}
Equation 2:
{P}=P_b \cdot \exp \left[\frac{-g_0 \cdot M \cdot (h-h_b)}{R^* \cdot T_b}\right]
where
Pb = Static pressure (pascals)
Tb = Standard temperature (kelvins)
Lb = Standard temperature lapse rate (kelvins per m)
h = Height above sea level (meters)
hb = Height at bottom of layer b (meters, e.g., h1 = 11,000 m)
R * = Universal gas constant: 8.31432 N·m / (mol·K)
g0 = Standard gravity (9.80665 m/s²)
M = Molar mass of Earth's air (0.0289644 kg/mol)
Or converted to English units:[4]
where
Pb = Static pressure (inches of mercury)
Tb = Standard temperature (kelvins)
Lb = Standard temperature lapse rate (kelvins per ft)
h = Height above sea level (feet)
hb = Height at bottom of layer b (feet, e.g., h1 = 36,089 ft)
R * = Universal gas constant (using feet and kelvins and gram moles: 8.9494596×104 kg·sq ft·s-2·K-1·kmol-1)
g0 = Standard gravity (32.17405 ft/s²)
M = Molar mass of Earth's air (0.0289644 kg/mol)
The value of subscript b ranges from 0 to 6 in accordance with each of seven successive layers of the atmosphere shown in the table below. In these equations, g0, M and R* are each single-valued constants, while P, L, T, and h are multivalued constants in accordance with the table below. (Note that according to the convention in this equation, L0, the tropospheric lapse rate, is negative.) It should be noted that the values used for M, g0, and R * are in accordance with the U.S. Standard Atmosphere, 1976, and that the value for R * in particular does not agree with standard values for this constant.[5] The reference value for Pb for b = 0 is the defined sea level value, P0 = 101325 pascals or 29.92126 inHg. Values of Pb of b = 1 through b = 6 are obtained from the application of the appropriate member of the pair equations 1 and 2 for the case when h = hb + 1.:[5]
Subscript b Height Above Sea Level Static Pressure Standard Temperature
(K) Temperature Lapse Rate
(m) (ft) (pascals) (inHg) (K/m) (K/ft)
0 0 0 101325 29.92126 288.15 -0.0065 -0.0019812
1 11,000 36,089 22632 6.683245 216.65 0.0 0.0
2 20,000 65,617 5474 1.616734 216.65 0.001 0.0003048
3 32,000 104,987 868 0.2563258 228.65 0.0028 0.00085344
4 47,000 154,199 110 0.0327506 270.65 0.0 0.0
5 51,000 167,323 66 0.01976704 270.65 -0.0028 -0.00085344
6 71,000 232,940 4 0.00116833 214.65 -0.002 -0.0006097
[edit] Local atmospheric pressure variation
Hurricane Wilma on 19 October 2005 – 88.2 kPa (12.79 psi) in eye
Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate. See pressure system for the effects of air pressure variations on weather.
Atmospheric pressure shows a diurnal (twice-daily) cycle caused by global atmospheric tides. This effect is strongest in tropical zones, with amplitude of a few millibars, and almost zero in polar areas. These variations have two superimposed cycles, a circadian (24 h) cycle and semi-circadian (12 h) cycle.
[edit] Atmospheric pressure based on height of water
Atmospheric pressure is often measured with a mercury barometer, and a height of approximately 760 millimetres (30 in) of mercury is often used to illustrate (and measure) atmospheric pressure. However, since mercury is not a substance that humans commonly come in contact with, water often provides a more intuitive way to visualize the pressure of one atmosphere.
One atmosphere (101.325 kPa or 14.7 lbf/sq in) is the amount of pressure that can lift water approximately 10.3 m (34 ft). Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). This is also the maximum height to which a column of water can be drawn up by suction.
Low pressures such as natural gas lines are sometimes specified in inches of water, typically written as w.c (water column) or W.G (inches water gauge). A typical gas using residential appliance is rated for a maximum of 14 w.c. which is approximately 0.5 atmosphere.
Non-professional barometers are generally aneroid barometers or strain gauge based. See Pressure measurement for a description of barometers.
[edit] Water's boiling point
Boiling water
Although water is generally considered to boil at 100 °C (212 °F), water actually boils when the vapor pressure is equal to the atmospheric pressure around the water.[6] Because of this, the boiling point of water is decreased in lower pressure and raised at higher pressure. This is why baking at elevations more than 3,500 ft (1,100 m) above sea level requires special baking directions.[7]
[edit] Effect on human health
Please help improve this section by expanding it. Further information might be found on the talk page. (November 2008)
Some studies have shown that atmospheric pressure can adversely affect human health, though many question whether the small natural variations caused, for example by weather fronts, are significant enough to affect humans.[8]
From Wikipedia, the free encyclopedia
Jump to: navigation, search
"Air pressure" redirects here. For the pressure of air in other systems, see pressure.
15 qyear average MSLP for JJA (top) and DJF (bottom).
Atmospheric pressure is sometimes defined as the force per unit area exerted against a surface by the weight of air above that surface at any given point in the Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. A column of air one square inch in cross-section, measured from sea level to the top of the atmosphere, would weigh approximately 14.7 lbf. The weight of a 1 m2 (11 sq ft) column of air would be about 100 kilonewtons (equivalent to a mass of 10.2 tonnes at the surface).
Contents
[hide]
* 1 Standard atmospheric pressure
* 2 Mean sea level pressure
* 3 Altitude atmospheric pressure variation
* 4 Calculating variation with altitude
* 5 Local atmospheric pressure variation
* 6 Atmospheric pressure based on height of water
* 7 Water's boiling point
* 8 Effect on human health
* 9 See also
* 10 Notes
* 11 References
* 12 External links
o 12.1 Experiments
[edit] Standard atmospheric pressure
The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being equal to 101.325 kPa. These other units are equivalent: 760 mmHg (torr), 29.92 inHg, 14.696 PSI, 1013.25 millibars. One standard atmosphere is standard pressure used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries.
In 1999, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of specifying the properties of substances, “the standard pressure” should be defined as precisely 100 kPa (≈750.01 torr) or 29.53 inHg rather than the 101.325 kPa value of “one standard atmosphere”.[1] This value is used as the standard pressure for the compressor and the pneumatic tool industries (ISO 2787).[2] (See also Standard temperature and pressure.) In the United States, compressed air flow is often measured in "standard cubic feet" per unit of time, where the "standard" means the equivalent quantity of moisture at standard temperature and pressure. However, this standard atmosphere is defined slightly differently: temperature = 20 °C (68 °F), air density = 1.225 kg/m³ (0.0765 lb/cu ft), altitude = sea level, and relative humidity = 20%. In the air conditioning industry, the standard is often temperature = 0 °C (32 °F) instead. For natural gas, the petroleum industry uses a standard temperature of 15.6 °C (60.1 °F), pressure 101.56 kPa (14.730 psi). (air pressure)
[edit] Mean sea level pressure
Mean sea level pressure (MSLP or QFF) is the pressure at sea level or (when measured at a given elevation on land) the station pressure reduced to sea level assuming an isothermal layer at the station temperature.
This is the pressure normally given in weather reports on radio, television, and newspapers or on the Internet. When barometers in the home are set to match the local weather reports, they measure pressure reduced to sea level, not the actual local atmospheric pressure. See Altimeter (barometer vs. absolute).
The reduction to sea level means that the normal range of fluctuations in pressure is the same for everyone. The pressures which are considered high pressure or low pressure do not depend on geographical location. This makes isobars on a weather map meaningful and useful tools.
Kollsman-type barometric aircraft altimeter as used in North America displaying an altitude of 80 ft (24 m).
The altimeter setting in aviation, set either QNH or QFE, is another atmospheric pressure reduced to sea level, but the method of making this reduction differs slightly.
QNH
The barometric altimeter setting which will cause the altimeter to read airfield elevation when on the airfield. In ISA temperature conditions the altimeter will read altitude above mean sea level in the vicinity of the airfield
QFE
The barometric altimeter setting which will cause an altimeter to read zero when at the reference datum of a particular airfield (generally a runway threshold). In ISA temperature conditions the altimeter will read height above the datum in the vicinity of the airfield.
QFE and QNH are arbitrary Q codes rather than abbreviations, but the mnemonics "Nautical Height" (for QNH) and "Field Elevation" (for QFE) are often used by pilots to distinguish them.
Average sea-level pressure is 101.325 kPa (1013.25 mbar) or 29.921 inches of mercury (inHg) or 760 millimeters (mmHg). In aviation weather reports (METAR), QNH is transmitted around the world in millibars or hectopascals (1 millibar = 1 hectopascal), except in the United States and in Canada where it is reported in inches (or hundredths of inches) of mercury. (The United States and Canada also report sea level pressure SLP, which is reduced to sea level by a different method, in the remarks section, not an internationally transmitted part of the code, in hectopascals or millibars [3]. However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals [1], while Environment Canada's standard unit of pressure is the same [2] [3].) In the weather code, three digits are all that is needed; decimal points and the one or two most significant digits are omitted: 1013.2 mbar or 101.32 kPa is transmitted as 132; 1000.0 mbar or 100.00 kPa is transmitted as 000; 998.7 mbar or 99.87 kPa is transmitted as 987; etc. The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1087.0 mbar. The lowest measurable sea-level pressure is found at the centers of tropical cyclones.
[edit] Altitude atmospheric pressure variation
This plastic bottle was closed at approximately 2,000 m (6,600 ft) altitude, then brought back to sea level. It was crushed by air pressure.
Pressure varies smoothly from the earth's surface to the top of the mesosphere. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. The following is a list of air pressures (as a fraction of one atmosphere) with the corresponding average altitudes. The table gives a rough idea of air pressure at various altitudes.
fraction of 1 atm average altitude
(m) (ft)
1 0 0
1/2 5,486 18,000
1/3 8,376 27,480
1/10 16,132 52,926
1/100 30,901 101,381
1/1000 48,467 159,013
1/10000 69,464 227,899
1/100000 86,282 283,076
[edit] Calculating variation with altitude
See also: Barometric formula
There are two different equations for computing the average pressure at various height regimes below 86 km (53 mi; 280,000 ft). Equation 1 is used when the value of standard temperature lapse rate is not equal to zero and equation 2 is used when standard temperature lapse rate equals zero.
Equation 1:
{P}=P_b \cdot \left[\frac{T_b}{T_b + L_b\cdot(h-h_b)}\right]^{\textstyle \frac{g_0 \cdot M}{R^* \cdot L_b}}
Equation 2:
{P}=P_b \cdot \exp \left[\frac{-g_0 \cdot M \cdot (h-h_b)}{R^* \cdot T_b}\right]
where
Pb = Static pressure (pascals)
Tb = Standard temperature (kelvins)
Lb = Standard temperature lapse rate (kelvins per m)
h = Height above sea level (meters)
hb = Height at bottom of layer b (meters, e.g., h1 = 11,000 m)
R * = Universal gas constant: 8.31432 N·m / (mol·K)
g0 = Standard gravity (9.80665 m/s²)
M = Molar mass of Earth's air (0.0289644 kg/mol)
Or converted to English units:[4]
where
Pb = Static pressure (inches of mercury)
Tb = Standard temperature (kelvins)
Lb = Standard temperature lapse rate (kelvins per ft)
h = Height above sea level (feet)
hb = Height at bottom of layer b (feet, e.g., h1 = 36,089 ft)
R * = Universal gas constant (using feet and kelvins and gram moles: 8.9494596×104 kg·sq ft·s-2·K-1·kmol-1)
g0 = Standard gravity (32.17405 ft/s²)
M = Molar mass of Earth's air (0.0289644 kg/mol)
The value of subscript b ranges from 0 to 6 in accordance with each of seven successive layers of the atmosphere shown in the table below. In these equations, g0, M and R* are each single-valued constants, while P, L, T, and h are multivalued constants in accordance with the table below. (Note that according to the convention in this equation, L0, the tropospheric lapse rate, is negative.) It should be noted that the values used for M, g0, and R * are in accordance with the U.S. Standard Atmosphere, 1976, and that the value for R * in particular does not agree with standard values for this constant.[5] The reference value for Pb for b = 0 is the defined sea level value, P0 = 101325 pascals or 29.92126 inHg. Values of Pb of b = 1 through b = 6 are obtained from the application of the appropriate member of the pair equations 1 and 2 for the case when h = hb + 1.:[5]
Subscript b Height Above Sea Level Static Pressure Standard Temperature
(K) Temperature Lapse Rate
(m) (ft) (pascals) (inHg) (K/m) (K/ft)
0 0 0 101325 29.92126 288.15 -0.0065 -0.0019812
1 11,000 36,089 22632 6.683245 216.65 0.0 0.0
2 20,000 65,617 5474 1.616734 216.65 0.001 0.0003048
3 32,000 104,987 868 0.2563258 228.65 0.0028 0.00085344
4 47,000 154,199 110 0.0327506 270.65 0.0 0.0
5 51,000 167,323 66 0.01976704 270.65 -0.0028 -0.00085344
6 71,000 232,940 4 0.00116833 214.65 -0.002 -0.0006097
[edit] Local atmospheric pressure variation
Hurricane Wilma on 19 October 2005 – 88.2 kPa (12.79 psi) in eye
Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate. See pressure system for the effects of air pressure variations on weather.
Atmospheric pressure shows a diurnal (twice-daily) cycle caused by global atmospheric tides. This effect is strongest in tropical zones, with amplitude of a few millibars, and almost zero in polar areas. These variations have two superimposed cycles, a circadian (24 h) cycle and semi-circadian (12 h) cycle.
[edit] Atmospheric pressure based on height of water
Atmospheric pressure is often measured with a mercury barometer, and a height of approximately 760 millimetres (30 in) of mercury is often used to illustrate (and measure) atmospheric pressure. However, since mercury is not a substance that humans commonly come in contact with, water often provides a more intuitive way to visualize the pressure of one atmosphere.
One atmosphere (101.325 kPa or 14.7 lbf/sq in) is the amount of pressure that can lift water approximately 10.3 m (34 ft). Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). This is also the maximum height to which a column of water can be drawn up by suction.
Low pressures such as natural gas lines are sometimes specified in inches of water, typically written as w.c (water column) or W.G (inches water gauge). A typical gas using residential appliance is rated for a maximum of 14 w.c. which is approximately 0.5 atmosphere.
Non-professional barometers are generally aneroid barometers or strain gauge based. See Pressure measurement for a description of barometers.
[edit] Water's boiling point
Boiling water
Although water is generally considered to boil at 100 °C (212 °F), water actually boils when the vapor pressure is equal to the atmospheric pressure around the water.[6] Because of this, the boiling point of water is decreased in lower pressure and raised at higher pressure. This is why baking at elevations more than 3,500 ft (1,100 m) above sea level requires special baking directions.[7]
[edit] Effect on human health
Please help improve this section by expanding it. Further information might be found on the talk page. (November 2008)
Some studies have shown that atmospheric pressure can adversely affect human health, though many question whether the small natural variations caused, for example by weather fronts, are significant enough to affect humans.[8]
pascal
Pascal (programming language)
From Wikipedia, the free encyclopedia
Jump to: navigation, search
This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed. (March 2009)
Pascal is an influential imperative and procedural programming language, designed in 1968/9 and published in 1970 by Niklaus Wirth as a small and efficient language intended to encourage good programming practices using structured programming and data structuring.
A derivative known as Object Pascal was designed for object oriented programming.
Pascal Paradigm imperative, structured
Appeared in 1970, last revised 1992
Designed by Niklaus Wirth
Typing discipline static, strong, safe
Major implementations CDC 6000, Pascal-P, PDP-11, PDP-10, IBM System/370, HP, GNU
Dialects UCSD, Borland, Turbo
Influenced by ALGOL
Influenced Modula-2, Oberon, Oberon-2, Component Pascal Ada, Object Pascal, Oxygene
Contents
[hide]
* 1 History
* 2 Brief description
* 3 Implementations
* 4 Language constructs
o 4.1 Hello world
o 4.2 Data types
o 4.3 Data structures
+ 4.3.1 Pointers
o 4.4 Control structures
o 4.5 Procedures and functions
* 5 Resources
o 5.1 Compilers and interpreters
* 6 Standards
o 6.1 Divisions
o 6.2 List of related standards
* 7 Reception
o 7.1 Criticism
+ 7.1.1 Reactions
* 8 See also
* 9 Further reading
* 10 External links
[edit] History
Pascal is based on the ALGOL programming language and named in honor of the French mathematician and philosopher Blaise Pascal. Wirth subsequently developed the Modula-2 and Oberon, languages similar to Pascal. Before, and leading up to Pascal, Wirth developed the language Euler, followed by Algol-W.
Initially, Pascal was largely, but not exclusively, intended to teach students structured programming. Generations of students have "cut their teeth" on Pascal as an introductory language in undergraduate courses. Variants of Pascal have also frequently been used for everything from research projects to PC games and embedded systems. Newer Pascal compilers exist which are widely used.
Pascal was the primary high-level language used for development in the Apple Lisa, and in the early years of the Mac; parts of the original Macintosh operating system were hand-translated into Motorola 68000 assembly language from the Pascal sources. The popular typesetting system TeX by Donald E. Knuth was written in WEB, the original literate programming system, based on DEC PDP-10 Pascal, while an application like Total Commander was written in Delphi (i.e. Object Pascal).
[edit] Brief description
Wirth's intention was to create an efficient language (regarding both compilation speed and generated code) based on so-called structured programming, a concept which had recently become popular. Pascal has its roots in the Algol 60 language, but also introduced concepts and mechanisms which (on top of Algol's scalars and arrays) enabled programmers to define their own complex (structured) datatypes, and also made it easier to build dynamic and recursive data structures such as lists, trees and graphs. Important features included for this were records, enumerations, subranges, dynamically allocated variables with associated pointers, and sets. To make this possible and meaningful, Pascal has a strong typing on all objects, which means that one type of data cannot be converted or interpreted as another without explicit conversions. Similar mechanisms are standard in many programming languages today. Other languages that influenced Pascal's development were COBOL, Simula 67, and Wirth's own Algol-W .
Pascal, like many scripting languages of today (but unlike most languages in the C family), allows nested procedure definitions to any level of depth, and also allows most kinds of definitions and declarations inside procedures and functions. This enables a very simple and coherent syntax where a complete program is syntactically nearly identical to a single procedure or function (except for the keyword itself, of course).
[edit] Implementations
The first Pascal compiler was designed in Zurich for the CDC 6000 series mainframe computer family. Niklaus Wirth reports that a first attempt to implement it in Fortran in 1969 was unsuccessful due to Fortran's inadequacy to express complex data structures. The second attempt was formulated in the Pascal language itself and was operational by mid-1970. Many Pascal compilers since have been similarly self-hosting, that is, the compiler is itself written in Pascal, and the compiler is usually capable of recompiling itself when new features are added to the language, or when the compiler is to be ported to a new environment. The GNU Pascal compiler is one notable exception, being written in C.
The first successful port of the CDC Pascal compiler to another mainframe was completed by Welsh and Quinn at the QUB in 1972. The target was the ICL 1900 series. This compiler in turn was the parent of the Pascal compiler for the ICS Multum minicomputer. The Multum port was developed – with a view to using Pascal as a systems programming language – by Findlay, Cupples, Cavouras and Davis, working at the Department of Computing Science in Glasgow University. It is thought that Multum Pascal, which was completed in the summer of 1973, may have been the first 16-bit implementation.
A completely new compiler was completed by Welsh et al. at QUB in 1977. It offered a source-language diagnostic feature (incorporating profiling, tracing and type-aware formatted postmortem dumps) that was implemented by Findlay and Watt at Glasgow University. This implementation was ported in 1980 to the ICL 2900 series by a team based at Southampton University and Glasgow University. The Standard Pascal Model Implementation was also based on this compiler, having been adapted, by Welsh and Hay at Manchester University in 1984, to check rigorously for conformity to the BSI 6192/ISO 7185 Standard and to generate code for a portable abstract machine.
The first Pascal compiler written in North America was constructed at the University of Illinois under Donald B. Gillies for the PDP-11 and generated native machine code. Pascal enjoyed great popularity throughout the 1970s and the 1980s.
In order to rapidly propagate the language, a compiler "porting kit" was created in Zurich that included a compiler that generated code for a "virtual" stack machine (i.e. code that lends itself to reasonably efficient interpretation), along with an interpreter for that code - the Pascal-P system. Although the SC (Stack Computer) code was primarily intended to be compiled into true machine code, at least one system, the notable UCSD implementation, utilized it to create the interpretive UCSD p-System. The P-system compilers were termed P1-P4, with P1 being the first version, and P4 being the last to come from Zurich.
The P4 compiler/interpreter can still be run and compiled on systems compatible with original Pascal. However, it only itself accepts a subset of the Pascal language. A version of P4 that accepts the full Pascal language and includes ISO 7185 compatibility was created and termed the P5 compiler, which is available in source form.
A version of the P4 compiler, which created native binaries, was released for the IBM System/370 mainframe computer by the Australian Atomic Energy Commission; it was called the "AAEC Pascal Compiler" after the abbreviation of the name of the Commission. A version of P4 from 1975-6 including source and binaries for the compiler and run-time library files for the PDP-10 mainframe may be downloaded from this link.
In the early 1980s, Watcom Pascal was developed, also for the IBM System 370.
IP Pascal was an implementation of the Pascal programming language using Micropolis DOS, but was moved rapidly to CP/M running on the Z80. It was moved to the 80386 machine types in 1994, and exists today as Windows/XP and Linux implementations. In 2008, the system was brought up to a new level and the resulting language termed "Pascaline" (after Pascal's calculator). It includes objects, namespace controls, dynamic arrays, along with many other extensions, and generally features the same functionality and type protection as C#. It is the only such implementation which is also compatible with the original Pascal implementation (which is standardized as ISO 7185).
In the early 1980s, UCSD Pascal was ported to the Apple II and Apple III computers to provide a structured alternative to the BASIC interpreters that came with the machines.
Apple Computer created its own Lisa Pascal for the Lisa Workshop in 1982 and ported this compiler to the Apple Macintosh and MPW in 1985. In 1985 Larry Tesler, in consultation with Niklaus Wirth, defined Object Pascal and these extensions were incorporated in both the Lisa Pascal and Mac Pascal compilers.
In the 1980s Anders Hejlsberg wrote the Blue Label Pascal compiler for the Nascom-2. A reimplementation of this compiler for the IBM PC was marketed under the names Compas Pascal and PolyPascal before it was acquired by Borland. Renamed to Turbo Pascal it became hugely popular, thanks in part to an aggressive pricing strategy and in part to having one of the first full-screen Integrated development environments. Additionally, it was written and highly optimized entirely in assembly language, making it smaller and faster than much of the competition. In 1986 Anders ported Turbo Pascal to the Macintosh and incorporated Apple's Object Pascal extensions into Turbo Pascal. These extensions were then added back into the PC version of Turbo Pascal for version 5.5.
The inexpensive Borland compiler had a large influence on the Pascal community that began concentrating mainly on the IBM PC in the late 1980s. Many PC hobbyists in search of a structured replacement for BASIC used this product. It also began adoption by professional developers. Around the same time a number of concepts were imported from C in order to let Pascal programmers use the C-based API of Microsoft Windows directly. These extensions included null-terminated strings, pointer arithmetic, function pointers, an address-of operator and unsafe typecasts.
However, Borland later decided it wanted more elaborate object-oriented features, and started over in Delphi using the Object Pascal draft standard proposed by Apple as a basis. (This Apple draft is still not a formal standard.) The first versions of the Delphi Programming Language were accordingly named Object Pascal. The main additions compared to the older OOP extensions were a reference-based object model, virtual constructors and destructors, and properties. Several other compilers also implement this dialect.
Turbo Pascal, and other derivatives with units or module concepts are modular languages. However, it does not provide a nested module concept or qualified import and export of specific symbols.
Super Pascal was a variant which added non-numeric labels, a return statement and expressions as names of types.
The universities of Zurich, Karlsruhe and Wuppertal have developed an EXtension for Scientific Computing (Pascal XSC), which provides a free solution for programming numerical computations with controlled precision.
In 2005, at the Web 2.0 conference, Morfik Technology introduced a tool which allowed the development of Web applications entirely written in Morfik Pascal. Morfik Pascal is a dialect of Object Pascal, very close to Delphi.
[edit] Language constructs
Pascal, in its original form, is a purely procedural language and includes the traditional array of Algol-like control structures with reserved words such as if, then, else, while, for, and so on. However, Pascal also has many data structuring facilities and other abstractions which were not included in the original Algol60, like type definitions, records, pointers, enumerations, and sets. Such constructs were in part inherited or inspired from Simula67, Algol68, Niklaus Wirth's own AlgolW and suggestions by C. A. R. Hoare.
[edit] Hello world
Pascal programs start with the program keyword with a list of external file descriptors as parameters; then follows the main statement block encapsulated by the begin and end keywords. Semicolons separate statements, and the full stop ends the whole program (or unit). Letter case is ignored in Pascal source. Some compilers, Turbo Pascal among them, have made the program keyword optional.
Here is an example of the source code in use for a very simple "Hello world" program:
Program HelloWorld(output);
begin
writeLn('Hello, World!')
end.
[edit] Data types
A type in Pascal, and in several other popular programming languages, defines a variable in such a way that it defines a range of values which the variable is capable of storing, and it also defines a set of operations that are permissible to be performed on variables of that type. The types and a very brief description follows;
Data type Range of values which the variable is capable of storing
integer Whole numbers from -32768 to 32767
byte The integers from 0 to 255
real Floating point numbers from 1E-38 to 1E+38
boolean Can only have the value TRUE or FALSE
char Any character in the ASCII character set
[edit] Data structures
Pascal's simple (atomic) types are real, integer, character, boolean and enumerations, a new type constructor introduced with Pascal:
var
r: Real;
i: Integer;
c: Char;
b: Boolean;
e: (apple, pear, banana, orange, lemon);
Subranges of any ordinal type (any simple type except real) can be made:
var
x: 1..10;
y: 'a'..'z';
z: pear..orange;
In contrast with other programming languages from its time, Pascal supports a set type:
var
set1: set of 1..10;
set2: set of 'a'..'z';
set3: set of pear..orange;
A set is fundamental concept for modern mathematics, and they may be used in a great many algorithms. Such a feature is highly useful and may be faster than an equivalent construct in a language that does not support sets. For example, for many Pascal compilers:
if i in [5..10] then
...
is faster, than
if (i>4) and (i<11) then
...
Types can be defined from other types using type declarations:
type
x = Integer;
y = x;
...
Further, complex types can be constructed from simple types:
type
a = Array [1..10] of Integer;
b = record
x: Integer;
y: Char
end;
c = File of a;
As shown in the example above, Pascal files are sequences of components. Every file has a buffer variable which is denoted by f^. The procedures get (for reading) and put (for writing) move the buffer variable to the next element. Read is introduced such that read(f, x) is the same as x:=f^; get(f);. Write is introduced such that write(f, x) is the same as f^ := x; put(f); The type text is predefined as file of char. While the buffer variable could be used to inspect the next character that would be used (check for a digit before reading an integer), this concept lead to serious problems with interactive programs with early implementations, but was solved later with the "lazy I/O" concept.
In Jensen & Wirth Pascal, strings are represented as packed arrays of chars; they therefore have fixed length and are usually space-padded. Some dialects have a custom string type.
[edit] Pointers
Pascal supports the use of pointers:
type
a = ^b;
b = record
a: Integer;
b: Char;
c: a
end;
var
pointertob: a;
Here the variable pointer_to_b is a pointer to the data type b, a record. Pointers can be used before they are declared. This is a forward declaration, an exception to the rule that things must be declared before they are used. To create a new record and assign the value 10 and character A to the fields a and b in the record, and to initialise the pointer c to nil, the commands would be:
new(pointer_to_b);
pointertob^.a := 10;
pointertob^.b := 'A';
pointertob^.c := nil;
...
This could also be done using the with statement, as follows
new(pointer_to_b);
with pointertob^ do
begin
a := 10;
b := 'A';
c := nil
end;
...
Note that inside of the scope of the with statement, the compiler knows that a and b refer to the subfields of the record pointer pointertob and not to the record b or the pointer type a.
Linked lists, stacks and queues can be created by including a pointer type field (c) in the record (see also nil and null (computer programming)).
[edit] Control structures
Pascal is a structured programming language, meaning that the flow of control is structured into standard statements, ideally without 'go to' commands.
while a <> b do writeln('Waiting');
if a > b then writeln('Condition met')
else writeln('Condition not met');
for i := 1 to 10 do writeln('Iteration: ', i:1);
repeat
a := a + 1
until a = 10;
case i of
0: write('zero');
1: write('one');
2: write('two')
end;
[edit] Procedures and functions
Pascal structures programs into procedures and functions.
program mine(output);
var i : integer;
procedure print(var j: integer);
function next(k: integer): integer;
begin
next := k + 1
end;
begin
writeln('The total is: ', j);
j := next(j)
end;
begin
i := 1;
while i <= 10 do print(i)
end.
Procedures and functions can nest to any depth, and the 'program' construct is the logical outermost block.
Each procedure or function can have its own declarations of goto labels, constants, types, variables, and other procedures and functions, which must all be in that order. This ordering requirement was originally intended to allow efficient single-pass compilation. However, in some dialects the strict ordering requirement of declaration sections is not required.
[edit] Resources
[edit] Compilers and interpreters
Several Pascal compilers and interpreters are available for the use of general public:
* Delphi is CodeGear's (formerly Borland) flagship RAD (Rapid Application Development) product. It uses the Object Pascal language (Dubbed the 'Delphi programming language' by Borland), descended from Pascal, to create applications for the windows platform. The .NET support that existed from D8 through D2005,D2006 and D2007 has been terminated, and replaced by a new language (Prism, which is rebranded Oxygene, see below) that is not fully backwards compatible. The most recent iteration of the win32 range (D2009) adds unicode and generics support. A version of Delphi (D2006), Turbo Delphi Explorer, is available for free download.
* Free Pascal (www.freepascal.org) is a multi-platform compiler written in Pascal (it is Self-hosting). It is aimed at providing a convenient and powerful compiler, both able to compile legacy applications and to be the means of developing new ones. It is distributed under the GNU GPL. Apart from compatibility modes for Turbo Pascal, Delphi and Mac Pascal, it also has its own procedural and object oriented syntax modes with support for extended features such as operator overloading. It supports many platforms and operating systems.
* Lazarus (lazarus.freepascal.org) is a Delphi-like visual cross-platform IDE for RAD (Rapid Application Development). Based on FreePascal, Lazarus is available for numerous platforms including Linux, FreeBSD, Mac OS X and Microsoft Windows.
* Dev-Pascal (Dev-Pascal) is a Pascal IDE that was designed in Borland Delphi and which supports both Free Pascal and GNU Pascal as backend. Contrary to its C++ sibling, it has not seen a significant release in years
* Oxygene (formerly known as Chrome) is a Next Generation Object Pascal compiler for the .NET and Mono Platforms. It was created and is sold by RemObjects Software, and recently by Codegear/Emarcadero as Prism It tries to carry the spirit of Pascal to .NET, but is not very compatible to other Pascals.
* Kylix was a descendant of Delphi, with support for the Linux operating system and an improved object library. The compiler and the IDE are available now for non-commercial use. The product is no longer supported by Borland.
* GNU Pascal Compiler (GPC) is the Pascal compiler of the GNU Compiler Collection (GCC). The compiler itself is written in C, the runtime library mostly in Pascal. Distributed freely under the GNU General Public License, it runs on many platforms and operating systems. It supports the ANSI/ISO standard languages and partial Borland/Turbo Pascal language support. One of the more painful omissions is the absence of a 100% TP compatible string type. Support for Borland Delphi and other language variations is quite limited, except maybe for Mac Pascal, the support for which is growing fast.
* Virtual Pascal was created by Vitaly Miryanov in 1995 as a native OS/2 compiler compatible with Borland Pascal syntax. Then, it had been commercially developed by fPrint, adding Win32 support, and in 2000 it became freeware. Today it can compile for Win32, OS/2 and Linux, and is mostly compatible with Borland Pascal and Delphi. Development on this compiler was canceled on April 4, 2005.
* P4 compiler, the basis for many subsequent Pascal-implemented-in-Pascal compilers, including the UCSD p-System. It implements a subset of full Pascal.
* P5 compiler, is an ISO 7185 (full Pascal) adaption of P4.
* Turbo Pascal was the dominant Pascal compiler for PCs during the 80s and early 90s, popular both because of its powerful extensions and extremely short compilation times. Turbo Pascal was compactly written and could compile, run, and debug all from memory without accessing disk. Slow floppy disk drives were common for programmers at the time, further magnifying Turbo Pascal's speed advantage. Currently, older versions of Turbo Pascal (up to 5.5) are available for free download from Borland's site.
* Turbo51 (turbo51.com) is a free Pascal compiler for the 8051 family of microcontrollers (uses Turbo Pascal 7 syntax)
* Dr. Pascal is an interpreter that runs Standard Pascal. Notable are the "visible execution" mode that shows a running program and its variables, and the extensive runtime error checking. Runs programs but does not produce a separate executable binary. Runs on MS-DOS, Windows in DOS window, and old Macintosh.
* Dr. Pascal's Extended Pascal Compiler tested on DOS, Windows 3.1, 95, 98, NT.
* IP Pascal Implements the language "Pascaline" (named after Pascal's calculator), which is a highly extended Pascal compatible with original Pascal according to ISO 7185. It features modules with namespace control, including parallel tasking modules with semaphores, objects, dynamic arrays of any dimensions that are allocated at runtime, overloads, overrides, and many other extensions. IP Pascal has a built-in portability library that is custom tailored to the Pascal language. For example, a standard text output application from 1970's original Pascal can be recompiled to work in a window and even have graphical constructs added.
* PocketStudio is a Pascal subset compiler/RAD targeting Palm / MC68xxx with some own extensions to assist interfacing with the Palm OS API.
* MIDletPascal - A Pascal compiler and IDE that generates small and fast Java bytecode specifically designed to create software for mobiles
* Vector Pascal Vector Pascal is a language targeted at SIMD instruction sets such as the MMX and the AMD 3d Now, supporting all Intel and AMD processors, as well as the Sony Playstation 2 Emotion Engine.
* Morfik Pascal allows the development of Web applications entirely written in Object Pascal (both server and browser side).
* web Pascal (www.codeide.com) is an online IDE and Pascal compiler.
* WDSibyl - Visual Development Environment and Pascal compiler for Win32 and OS/2
* PP Compiler, a compiler for Palm OS that runs directly on the handheld computer
* CDC 6000 Pascal compiler The source code for the first (CDC 6000) Pascal compiler.
* Pascal-S - "Pascal-S: A Subset and Its Implementation", N. Wirth in Pascal - The Language and Its Implementation, by D.W. Barron, Wiley 1979.
A very extensive list can be found on Pascaland. The site is in French, but it is basically a list with URLs to compilers; there is little barrier for non-Francophones. The site, Pascal Central, a Mac centric Pascal info and advocacy site with a rich collection of article archives, plus links to many compilers and tutorials, may also be of interest.
[edit] Standards
In 1983, the language was standardized, in the international standard ISO/IEC 7185, as well as several local country specific standards, including the American ANSI/IEEE770X3.97-1983, and ISO 7185:1983. These two standards differed only in that the ISO standard included a "level 1" extension for conformant arrays, where ANSI did not allow for this extension to the original (Wirth version) language. In 1989, ISO 7185 was revised (ISO 7185:1990) to correct various errors and ambiguities found in the original document.
In 1990, an extended Pascal standard was created as ISO/IEC 10206. In 1993 the ANSI standard was replaced by the ANSI organization with a "pointer" to the ISO 7185:1990 standard, effectively ending its status as a different standard.
The ISO 7185 was stated to be a clarification of Wirth's 1974 language as detailed by the User Manual and Report [Jensen and Wirth], but was also notable for adding "Conformant Array Parameters" as a level 1 to the standard, level 0 being Pascal without Conformant Arrays.
Note that Niklaus Wirth himself referred to the 1974 language as "the Standard", for example, to differentiate it from the machine specific features of the CDC 6000 compiler. This language was documented in "The Pascal Report", the second part of the "Pascal users manual and report".
On the large machines (mainframes and minicomputers) Pascal originated on, the standards were generally followed. On the IBM-PC, they were not. On IBM-PCs, the Borland standards Turbo Pascal and Delphi have the greatest number of users. Thus, it is typically important to understand whether a particular implementation corresponds to the original Pascal language, or a Borland dialect of it.
The IBM-PC versions of the language began to differ with the advent of UCSD Pascal, an interpreted implementation that featured several extensions to the language, along with several omissions and changes. Many UCSD language features survive today, including in Borland's dialect.
[edit] Divisions
Niklaus Wirth's Zurich version of Pascal was issued outside of ETH in two basic forms, the CDC 6000 compiler source, and a porting kit called Pascal-P system. The Pascal-P compiler left out several features of the full language. For example, procedures and functions used as parameters, undiscriminated variant records, packing, dispose, interprocedural gotos and other features of the full compiler were omitted.
UCSD Pascal, under Professor Kenneth Bowles, was based on the Pascal-P2 kit, and consequently shared several of the Pascal-P language restrictions. UCSD Pascal was later adopted as Apple Pascal, and continued through several versions there. Although UCSD Pascal actually expanded the subset Pascal in the Pascal-P kit by adding back standard Pascal constructs, it was still not a complete standard installation of Pascal.
Borland's Turbo Pascal, written by Anders Hejlsberg was written in assembly language independent of UCSD or the Zurich compilers. However, it adopted much of the same subset and extensions as the UCSD compiler. This is probably because the UCSD system was the most common Pascal system suitable for developing applications on the resource-limited microprocessor systems available at that time.
[edit] List of related standards
* ISO 8651-2:1988 Information processing systems -- Computer graphics -- Graphical Kernel System (GKS) language bindings -- Part 2: Pascal
[edit] Reception
Pascal generated a wide variety of responses in the computing community, both critical and complimentary.
[edit] Criticism
While very popular (although more so in the 1980s and early 1990s than now), early versions of Pascal have been widely criticized for being unsuitable for "serious" use outside of teaching. Brian Kernighan, who popularized the C programming language, outlined his most notable criticisms of Pascal as early as 1981, in his paper Why Pascal Is Not My Favorite Programming Language. On the other hand, many major development efforts in the 1980s, such as for the Apple Lisa and Macintosh, heavily depended on Pascal (to the point where the C interface for the Macintosh operating system API had to deal in Pascal data types).
[edit] Reactions
In the decades since then, Pascal in fact has been continuing to evolve, and most of Kernighan's points do not apply to current implementations. Unfortunately, just as Kernighan predicted in his article, most of the extensions to fix these issues were incompatible from compiler to compiler. In the last decade, however, the varieties seem to have condensed into two categories, ISO and Borland like, a better eventual outcome than Kernighan foresaw.[original research?]
Although Kernighan decried Pascal's lack of type escapes ("there is no escape" from "Why Pascal is not my Favorite Programming language"), the uncontrolled use of pointers and type escapes have become highly criticized features in their own right, and the languages Java, C# and others feature a sharp turn-around to the Pascal point of view. What these languages call "managed pointers" were in fact foreseen by Wirth with the creation of Pascal.
Based on his experience with Pascal (and earlier with ALGOL) Niklaus Wirth developed several more programming languages: Modula, Modula-2 and Oberon. These languages address some criticisms of Pascal, are intended for different user populations, and so on, but none has had the widespread impact on computer science and computer users as has Pascal, nor has any yet met with similar commercial success.
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Pascal is an influential imperative and procedural programming language, designed in 1968/9 and published in 1970 by Niklaus Wirth as a small and efficient language intended to encourage good programming practices using structured programming and data structuring.
A derivative known as Object Pascal was designed for object oriented programming.
Pascal Paradigm imperative, structured
Appeared in 1970, last revised 1992
Designed by Niklaus Wirth
Typing discipline static, strong, safe
Major implementations CDC 6000, Pascal-P, PDP-11, PDP-10, IBM System/370, HP, GNU
Dialects UCSD, Borland, Turbo
Influenced by ALGOL
Influenced Modula-2, Oberon, Oberon-2, Component Pascal Ada, Object Pascal, Oxygene
Contents
[hide]
* 1 History
* 2 Brief description
* 3 Implementations
* 4 Language constructs
o 4.1 Hello world
o 4.2 Data types
o 4.3 Data structures
+ 4.3.1 Pointers
o 4.4 Control structures
o 4.5 Procedures and functions
* 5 Resources
o 5.1 Compilers and interpreters
* 6 Standards
o 6.1 Divisions
o 6.2 List of related standards
* 7 Reception
o 7.1 Criticism
+ 7.1.1 Reactions
* 8 See also
* 9 Further reading
* 10 External links
[edit] History
Pascal is based on the ALGOL programming language and named in honor of the French mathematician and philosopher Blaise Pascal. Wirth subsequently developed the Modula-2 and Oberon, languages similar to Pascal. Before, and leading up to Pascal, Wirth developed the language Euler, followed by Algol-W.
Initially, Pascal was largely, but not exclusively, intended to teach students structured programming. Generations of students have "cut their teeth" on Pascal as an introductory language in undergraduate courses. Variants of Pascal have also frequently been used for everything from research projects to PC games and embedded systems. Newer Pascal compilers exist which are widely used.
Pascal was the primary high-level language used for development in the Apple Lisa, and in the early years of the Mac; parts of the original Macintosh operating system were hand-translated into Motorola 68000 assembly language from the Pascal sources. The popular typesetting system TeX by Donald E. Knuth was written in WEB, the original literate programming system, based on DEC PDP-10 Pascal, while an application like Total Commander was written in Delphi (i.e. Object Pascal).
[edit] Brief description
Wirth's intention was to create an efficient language (regarding both compilation speed and generated code) based on so-called structured programming, a concept which had recently become popular. Pascal has its roots in the Algol 60 language, but also introduced concepts and mechanisms which (on top of Algol's scalars and arrays) enabled programmers to define their own complex (structured) datatypes, and also made it easier to build dynamic and recursive data structures such as lists, trees and graphs. Important features included for this were records, enumerations, subranges, dynamically allocated variables with associated pointers, and sets. To make this possible and meaningful, Pascal has a strong typing on all objects, which means that one type of data cannot be converted or interpreted as another without explicit conversions. Similar mechanisms are standard in many programming languages today. Other languages that influenced Pascal's development were COBOL, Simula 67, and Wirth's own Algol-W .
Pascal, like many scripting languages of today (but unlike most languages in the C family), allows nested procedure definitions to any level of depth, and also allows most kinds of definitions and declarations inside procedures and functions. This enables a very simple and coherent syntax where a complete program is syntactically nearly identical to a single procedure or function (except for the keyword itself, of course).
[edit] Implementations
The first Pascal compiler was designed in Zurich for the CDC 6000 series mainframe computer family. Niklaus Wirth reports that a first attempt to implement it in Fortran in 1969 was unsuccessful due to Fortran's inadequacy to express complex data structures. The second attempt was formulated in the Pascal language itself and was operational by mid-1970. Many Pascal compilers since have been similarly self-hosting, that is, the compiler is itself written in Pascal, and the compiler is usually capable of recompiling itself when new features are added to the language, or when the compiler is to be ported to a new environment. The GNU Pascal compiler is one notable exception, being written in C.
The first successful port of the CDC Pascal compiler to another mainframe was completed by Welsh and Quinn at the QUB in 1972. The target was the ICL 1900 series. This compiler in turn was the parent of the Pascal compiler for the ICS Multum minicomputer. The Multum port was developed – with a view to using Pascal as a systems programming language – by Findlay, Cupples, Cavouras and Davis, working at the Department of Computing Science in Glasgow University. It is thought that Multum Pascal, which was completed in the summer of 1973, may have been the first 16-bit implementation.
A completely new compiler was completed by Welsh et al. at QUB in 1977. It offered a source-language diagnostic feature (incorporating profiling, tracing and type-aware formatted postmortem dumps) that was implemented by Findlay and Watt at Glasgow University. This implementation was ported in 1980 to the ICL 2900 series by a team based at Southampton University and Glasgow University. The Standard Pascal Model Implementation was also based on this compiler, having been adapted, by Welsh and Hay at Manchester University in 1984, to check rigorously for conformity to the BSI 6192/ISO 7185 Standard and to generate code for a portable abstract machine.
The first Pascal compiler written in North America was constructed at the University of Illinois under Donald B. Gillies for the PDP-11 and generated native machine code. Pascal enjoyed great popularity throughout the 1970s and the 1980s.
In order to rapidly propagate the language, a compiler "porting kit" was created in Zurich that included a compiler that generated code for a "virtual" stack machine (i.e. code that lends itself to reasonably efficient interpretation), along with an interpreter for that code - the Pascal-P system. Although the SC (Stack Computer) code was primarily intended to be compiled into true machine code, at least one system, the notable UCSD implementation, utilized it to create the interpretive UCSD p-System. The P-system compilers were termed P1-P4, with P1 being the first version, and P4 being the last to come from Zurich.
The P4 compiler/interpreter can still be run and compiled on systems compatible with original Pascal. However, it only itself accepts a subset of the Pascal language. A version of P4 that accepts the full Pascal language and includes ISO 7185 compatibility was created and termed the P5 compiler, which is available in source form.
A version of the P4 compiler, which created native binaries, was released for the IBM System/370 mainframe computer by the Australian Atomic Energy Commission; it was called the "AAEC Pascal Compiler" after the abbreviation of the name of the Commission. A version of P4 from 1975-6 including source and binaries for the compiler and run-time library files for the PDP-10 mainframe may be downloaded from this link.
In the early 1980s, Watcom Pascal was developed, also for the IBM System 370.
IP Pascal was an implementation of the Pascal programming language using Micropolis DOS, but was moved rapidly to CP/M running on the Z80. It was moved to the 80386 machine types in 1994, and exists today as Windows/XP and Linux implementations. In 2008, the system was brought up to a new level and the resulting language termed "Pascaline" (after Pascal's calculator). It includes objects, namespace controls, dynamic arrays, along with many other extensions, and generally features the same functionality and type protection as C#. It is the only such implementation which is also compatible with the original Pascal implementation (which is standardized as ISO 7185).
In the early 1980s, UCSD Pascal was ported to the Apple II and Apple III computers to provide a structured alternative to the BASIC interpreters that came with the machines.
Apple Computer created its own Lisa Pascal for the Lisa Workshop in 1982 and ported this compiler to the Apple Macintosh and MPW in 1985. In 1985 Larry Tesler, in consultation with Niklaus Wirth, defined Object Pascal and these extensions were incorporated in both the Lisa Pascal and Mac Pascal compilers.
In the 1980s Anders Hejlsberg wrote the Blue Label Pascal compiler for the Nascom-2. A reimplementation of this compiler for the IBM PC was marketed under the names Compas Pascal and PolyPascal before it was acquired by Borland. Renamed to Turbo Pascal it became hugely popular, thanks in part to an aggressive pricing strategy and in part to having one of the first full-screen Integrated development environments. Additionally, it was written and highly optimized entirely in assembly language, making it smaller and faster than much of the competition. In 1986 Anders ported Turbo Pascal to the Macintosh and incorporated Apple's Object Pascal extensions into Turbo Pascal. These extensions were then added back into the PC version of Turbo Pascal for version 5.5.
The inexpensive Borland compiler had a large influence on the Pascal community that began concentrating mainly on the IBM PC in the late 1980s. Many PC hobbyists in search of a structured replacement for BASIC used this product. It also began adoption by professional developers. Around the same time a number of concepts were imported from C in order to let Pascal programmers use the C-based API of Microsoft Windows directly. These extensions included null-terminated strings, pointer arithmetic, function pointers, an address-of operator and unsafe typecasts.
However, Borland later decided it wanted more elaborate object-oriented features, and started over in Delphi using the Object Pascal draft standard proposed by Apple as a basis. (This Apple draft is still not a formal standard.) The first versions of the Delphi Programming Language were accordingly named Object Pascal. The main additions compared to the older OOP extensions were a reference-based object model, virtual constructors and destructors, and properties. Several other compilers also implement this dialect.
Turbo Pascal, and other derivatives with units or module concepts are modular languages. However, it does not provide a nested module concept or qualified import and export of specific symbols.
Super Pascal was a variant which added non-numeric labels, a return statement and expressions as names of types.
The universities of Zurich, Karlsruhe and Wuppertal have developed an EXtension for Scientific Computing (Pascal XSC), which provides a free solution for programming numerical computations with controlled precision.
In 2005, at the Web 2.0 conference, Morfik Technology introduced a tool which allowed the development of Web applications entirely written in Morfik Pascal. Morfik Pascal is a dialect of Object Pascal, very close to Delphi.
[edit] Language constructs
Pascal, in its original form, is a purely procedural language and includes the traditional array of Algol-like control structures with reserved words such as if, then, else, while, for, and so on. However, Pascal also has many data structuring facilities and other abstractions which were not included in the original Algol60, like type definitions, records, pointers, enumerations, and sets. Such constructs were in part inherited or inspired from Simula67, Algol68, Niklaus Wirth's own AlgolW and suggestions by C. A. R. Hoare.
[edit] Hello world
Pascal programs start with the program keyword with a list of external file descriptors as parameters; then follows the main statement block encapsulated by the begin and end keywords. Semicolons separate statements, and the full stop ends the whole program (or unit). Letter case is ignored in Pascal source. Some compilers, Turbo Pascal among them, have made the program keyword optional.
Here is an example of the source code in use for a very simple "Hello world" program:
Program HelloWorld(output);
begin
writeLn('Hello, World!')
end.
[edit] Data types
A type in Pascal, and in several other popular programming languages, defines a variable in such a way that it defines a range of values which the variable is capable of storing, and it also defines a set of operations that are permissible to be performed on variables of that type. The types and a very brief description follows;
Data type Range of values which the variable is capable of storing
integer Whole numbers from -32768 to 32767
byte The integers from 0 to 255
real Floating point numbers from 1E-38 to 1E+38
boolean Can only have the value TRUE or FALSE
char Any character in the ASCII character set
[edit] Data structures
Pascal's simple (atomic) types are real, integer, character, boolean and enumerations, a new type constructor introduced with Pascal:
var
r: Real;
i: Integer;
c: Char;
b: Boolean;
e: (apple, pear, banana, orange, lemon);
Subranges of any ordinal type (any simple type except real) can be made:
var
x: 1..10;
y: 'a'..'z';
z: pear..orange;
In contrast with other programming languages from its time, Pascal supports a set type:
var
set1: set of 1..10;
set2: set of 'a'..'z';
set3: set of pear..orange;
A set is fundamental concept for modern mathematics, and they may be used in a great many algorithms. Such a feature is highly useful and may be faster than an equivalent construct in a language that does not support sets. For example, for many Pascal compilers:
if i in [5..10] then
...
is faster, than
if (i>4) and (i<11) then
...
Types can be defined from other types using type declarations:
type
x = Integer;
y = x;
...
Further, complex types can be constructed from simple types:
type
a = Array [1..10] of Integer;
b = record
x: Integer;
y: Char
end;
c = File of a;
As shown in the example above, Pascal files are sequences of components. Every file has a buffer variable which is denoted by f^. The procedures get (for reading) and put (for writing) move the buffer variable to the next element. Read is introduced such that read(f, x) is the same as x:=f^; get(f);. Write is introduced such that write(f, x) is the same as f^ := x; put(f); The type text is predefined as file of char. While the buffer variable could be used to inspect the next character that would be used (check for a digit before reading an integer), this concept lead to serious problems with interactive programs with early implementations, but was solved later with the "lazy I/O" concept.
In Jensen & Wirth Pascal, strings are represented as packed arrays of chars; they therefore have fixed length and are usually space-padded. Some dialects have a custom string type.
[edit] Pointers
Pascal supports the use of pointers:
type
a = ^b;
b = record
a: Integer;
b: Char;
c: a
end;
var
pointertob: a;
Here the variable pointer_to_b is a pointer to the data type b, a record. Pointers can be used before they are declared. This is a forward declaration, an exception to the rule that things must be declared before they are used. To create a new record and assign the value 10 and character A to the fields a and b in the record, and to initialise the pointer c to nil, the commands would be:
new(pointer_to_b);
pointertob^.a := 10;
pointertob^.b := 'A';
pointertob^.c := nil;
...
This could also be done using the with statement, as follows
new(pointer_to_b);
with pointertob^ do
begin
a := 10;
b := 'A';
c := nil
end;
...
Note that inside of the scope of the with statement, the compiler knows that a and b refer to the subfields of the record pointer pointertob and not to the record b or the pointer type a.
Linked lists, stacks and queues can be created by including a pointer type field (c) in the record (see also nil and null (computer programming)).
[edit] Control structures
Pascal is a structured programming language, meaning that the flow of control is structured into standard statements, ideally without 'go to' commands.
while a <> b do writeln('Waiting');
if a > b then writeln('Condition met')
else writeln('Condition not met');
for i := 1 to 10 do writeln('Iteration: ', i:1);
repeat
a := a + 1
until a = 10;
case i of
0: write('zero');
1: write('one');
2: write('two')
end;
[edit] Procedures and functions
Pascal structures programs into procedures and functions.
program mine(output);
var i : integer;
procedure print(var j: integer);
function next(k: integer): integer;
begin
next := k + 1
end;
begin
writeln('The total is: ', j);
j := next(j)
end;
begin
i := 1;
while i <= 10 do print(i)
end.
Procedures and functions can nest to any depth, and the 'program' construct is the logical outermost block.
Each procedure or function can have its own declarations of goto labels, constants, types, variables, and other procedures and functions, which must all be in that order. This ordering requirement was originally intended to allow efficient single-pass compilation. However, in some dialects the strict ordering requirement of declaration sections is not required.
[edit] Resources
[edit] Compilers and interpreters
Several Pascal compilers and interpreters are available for the use of general public:
* Delphi is CodeGear's (formerly Borland) flagship RAD (Rapid Application Development) product. It uses the Object Pascal language (Dubbed the 'Delphi programming language' by Borland), descended from Pascal, to create applications for the windows platform. The .NET support that existed from D8 through D2005,D2006 and D2007 has been terminated, and replaced by a new language (Prism, which is rebranded Oxygene, see below) that is not fully backwards compatible. The most recent iteration of the win32 range (D2009) adds unicode and generics support. A version of Delphi (D2006), Turbo Delphi Explorer, is available for free download.
* Free Pascal (www.freepascal.org) is a multi-platform compiler written in Pascal (it is Self-hosting). It is aimed at providing a convenient and powerful compiler, both able to compile legacy applications and to be the means of developing new ones. It is distributed under the GNU GPL. Apart from compatibility modes for Turbo Pascal, Delphi and Mac Pascal, it also has its own procedural and object oriented syntax modes with support for extended features such as operator overloading. It supports many platforms and operating systems.
* Lazarus (lazarus.freepascal.org) is a Delphi-like visual cross-platform IDE for RAD (Rapid Application Development). Based on FreePascal, Lazarus is available for numerous platforms including Linux, FreeBSD, Mac OS X and Microsoft Windows.
* Dev-Pascal (Dev-Pascal) is a Pascal IDE that was designed in Borland Delphi and which supports both Free Pascal and GNU Pascal as backend. Contrary to its C++ sibling, it has not seen a significant release in years
* Oxygene (formerly known as Chrome) is a Next Generation Object Pascal compiler for the .NET and Mono Platforms. It was created and is sold by RemObjects Software, and recently by Codegear/Emarcadero as Prism It tries to carry the spirit of Pascal to .NET, but is not very compatible to other Pascals.
* Kylix was a descendant of Delphi, with support for the Linux operating system and an improved object library. The compiler and the IDE are available now for non-commercial use. The product is no longer supported by Borland.
* GNU Pascal Compiler (GPC) is the Pascal compiler of the GNU Compiler Collection (GCC). The compiler itself is written in C, the runtime library mostly in Pascal. Distributed freely under the GNU General Public License, it runs on many platforms and operating systems. It supports the ANSI/ISO standard languages and partial Borland/Turbo Pascal language support. One of the more painful omissions is the absence of a 100% TP compatible string type. Support for Borland Delphi and other language variations is quite limited, except maybe for Mac Pascal, the support for which is growing fast.
* Virtual Pascal was created by Vitaly Miryanov in 1995 as a native OS/2 compiler compatible with Borland Pascal syntax. Then, it had been commercially developed by fPrint, adding Win32 support, and in 2000 it became freeware. Today it can compile for Win32, OS/2 and Linux, and is mostly compatible with Borland Pascal and Delphi. Development on this compiler was canceled on April 4, 2005.
* P4 compiler, the basis for many subsequent Pascal-implemented-in-Pascal compilers, including the UCSD p-System. It implements a subset of full Pascal.
* P5 compiler, is an ISO 7185 (full Pascal) adaption of P4.
* Turbo Pascal was the dominant Pascal compiler for PCs during the 80s and early 90s, popular both because of its powerful extensions and extremely short compilation times. Turbo Pascal was compactly written and could compile, run, and debug all from memory without accessing disk. Slow floppy disk drives were common for programmers at the time, further magnifying Turbo Pascal's speed advantage. Currently, older versions of Turbo Pascal (up to 5.5) are available for free download from Borland's site.
* Turbo51 (turbo51.com) is a free Pascal compiler for the 8051 family of microcontrollers (uses Turbo Pascal 7 syntax)
* Dr. Pascal is an interpreter that runs Standard Pascal. Notable are the "visible execution" mode that shows a running program and its variables, and the extensive runtime error checking. Runs programs but does not produce a separate executable binary. Runs on MS-DOS, Windows in DOS window, and old Macintosh.
* Dr. Pascal's Extended Pascal Compiler tested on DOS, Windows 3.1, 95, 98, NT.
* IP Pascal Implements the language "Pascaline" (named after Pascal's calculator), which is a highly extended Pascal compatible with original Pascal according to ISO 7185. It features modules with namespace control, including parallel tasking modules with semaphores, objects, dynamic arrays of any dimensions that are allocated at runtime, overloads, overrides, and many other extensions. IP Pascal has a built-in portability library that is custom tailored to the Pascal language. For example, a standard text output application from 1970's original Pascal can be recompiled to work in a window and even have graphical constructs added.
* PocketStudio is a Pascal subset compiler/RAD targeting Palm / MC68xxx with some own extensions to assist interfacing with the Palm OS API.
* MIDletPascal - A Pascal compiler and IDE that generates small and fast Java bytecode specifically designed to create software for mobiles
* Vector Pascal Vector Pascal is a language targeted at SIMD instruction sets such as the MMX and the AMD 3d Now, supporting all Intel and AMD processors, as well as the Sony Playstation 2 Emotion Engine.
* Morfik Pascal allows the development of Web applications entirely written in Object Pascal (both server and browser side).
* web Pascal (www.codeide.com) is an online IDE and Pascal compiler.
* WDSibyl - Visual Development Environment and Pascal compiler for Win32 and OS/2
* PP Compiler, a compiler for Palm OS that runs directly on the handheld computer
* CDC 6000 Pascal compiler The source code for the first (CDC 6000) Pascal compiler.
* Pascal-S - "Pascal-S: A Subset and Its Implementation", N. Wirth in Pascal - The Language and Its Implementation, by D.W. Barron, Wiley 1979.
A very extensive list can be found on Pascaland. The site is in French, but it is basically a list with URLs to compilers; there is little barrier for non-Francophones. The site, Pascal Central, a Mac centric Pascal info and advocacy site with a rich collection of article archives, plus links to many compilers and tutorials, may also be of interest.
[edit] Standards
In 1983, the language was standardized, in the international standard ISO/IEC 7185, as well as several local country specific standards, including the American ANSI/IEEE770X3.97-1983, and ISO 7185:1983. These two standards differed only in that the ISO standard included a "level 1" extension for conformant arrays, where ANSI did not allow for this extension to the original (Wirth version) language. In 1989, ISO 7185 was revised (ISO 7185:1990) to correct various errors and ambiguities found in the original document.
In 1990, an extended Pascal standard was created as ISO/IEC 10206. In 1993 the ANSI standard was replaced by the ANSI organization with a "pointer" to the ISO 7185:1990 standard, effectively ending its status as a different standard.
The ISO 7185 was stated to be a clarification of Wirth's 1974 language as detailed by the User Manual and Report [Jensen and Wirth], but was also notable for adding "Conformant Array Parameters" as a level 1 to the standard, level 0 being Pascal without Conformant Arrays.
Note that Niklaus Wirth himself referred to the 1974 language as "the Standard", for example, to differentiate it from the machine specific features of the CDC 6000 compiler. This language was documented in "The Pascal Report", the second part of the "Pascal users manual and report".
On the large machines (mainframes and minicomputers) Pascal originated on, the standards were generally followed. On the IBM-PC, they were not. On IBM-PCs, the Borland standards Turbo Pascal and Delphi have the greatest number of users. Thus, it is typically important to understand whether a particular implementation corresponds to the original Pascal language, or a Borland dialect of it.
The IBM-PC versions of the language began to differ with the advent of UCSD Pascal, an interpreted implementation that featured several extensions to the language, along with several omissions and changes. Many UCSD language features survive today, including in Borland's dialect.
[edit] Divisions
Niklaus Wirth's Zurich version of Pascal was issued outside of ETH in two basic forms, the CDC 6000 compiler source, and a porting kit called Pascal-P system. The Pascal-P compiler left out several features of the full language. For example, procedures and functions used as parameters, undiscriminated variant records, packing, dispose, interprocedural gotos and other features of the full compiler were omitted.
UCSD Pascal, under Professor Kenneth Bowles, was based on the Pascal-P2 kit, and consequently shared several of the Pascal-P language restrictions. UCSD Pascal was later adopted as Apple Pascal, and continued through several versions there. Although UCSD Pascal actually expanded the subset Pascal in the Pascal-P kit by adding back standard Pascal constructs, it was still not a complete standard installation of Pascal.
Borland's Turbo Pascal, written by Anders Hejlsberg was written in assembly language independent of UCSD or the Zurich compilers. However, it adopted much of the same subset and extensions as the UCSD compiler. This is probably because the UCSD system was the most common Pascal system suitable for developing applications on the resource-limited microprocessor systems available at that time.
[edit] List of related standards
* ISO 8651-2:1988 Information processing systems -- Computer graphics -- Graphical Kernel System (GKS) language bindings -- Part 2: Pascal
[edit] Reception
Pascal generated a wide variety of responses in the computing community, both critical and complimentary.
[edit] Criticism
While very popular (although more so in the 1980s and early 1990s than now), early versions of Pascal have been widely criticized for being unsuitable for "serious" use outside of teaching. Brian Kernighan, who popularized the C programming language, outlined his most notable criticisms of Pascal as early as 1981, in his paper Why Pascal Is Not My Favorite Programming Language. On the other hand, many major development efforts in the 1980s, such as for the Apple Lisa and Macintosh, heavily depended on Pascal (to the point where the C interface for the Macintosh operating system API had to deal in Pascal data types).
[edit] Reactions
In the decades since then, Pascal in fact has been continuing to evolve, and most of Kernighan's points do not apply to current implementations. Unfortunately, just as Kernighan predicted in his article, most of the extensions to fix these issues were incompatible from compiler to compiler. In the last decade, however, the varieties seem to have condensed into two categories, ISO and Borland like, a better eventual outcome than Kernighan foresaw.[original research?]
Although Kernighan decried Pascal's lack of type escapes ("there is no escape" from "Why Pascal is not my Favorite Programming language"), the uncontrolled use of pointers and type escapes have become highly criticized features in their own right, and the languages Java, C# and others feature a sharp turn-around to the Pascal point of view. What these languages call "managed pointers" were in fact foreseen by Wirth with the creation of Pascal.
Based on his experience with Pascal (and earlier with ALGOL) Niklaus Wirth developed several more programming languages: Modula, Modula-2 and Oberon. These languages address some criticisms of Pascal, are intended for different user populations, and so on, but none has had the widespread impact on computer science and computer users as has Pascal, nor has any yet met with similar commercial success.
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