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Speed
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This article is about the property of moving bodies. For other uses, see Speed (disambiguation).

Speed is the rate of motion, or equivalently the rate of change of distance.

Speed is a scalar quantity with dimensions length/time; the equivalent vector quantity to speed is velocity. Speed is measured in the same physical units of measurement as velocity, but does not contain the element of direction that velocity has. Speed is thus the magnitude component of velocity.

In mathematical notation, if an object traveling at constant speed moves a distance x in time t, its speed, denoted by v, is simply given by

v = \frac {x}{t}.

In many situations, objects do not move at a constant speed. For example, if a car goes 60 miles in 2 hours, its average speed during that time is 30 miles per hour, but its instantaneous speed may have varied. For an object which is accelerating or decelerating, the instantaneous speed is given by

v = \left|\frac {dx}{dt}\right|,

where dx is the distance it travels in a very short period of time dt. If the object travels a total distance x in time t, its average speed over that time is given by

\bar{v} = \left|\frac {x}{t}\right|.

Contents
[hide]

* 1 Units
* 2 Examples of different speeds
* 3 See also
* 4 References
* 5 External links

[edit] Units

Units of speed include:

* meters per second, (symbol ms-1; m/s), the SI derived unit
* kilometers per hour, (symbol km/h)
* miles per hour, (symbol mph)
* knots (nautical miles per hour, symbol kt)
* Mach number, speed divided by the speed of sound
* speed of light in vacuum (symbol c) is one of the natural units

c = 299,792,458 ms-1

Important conversions between units of speed include:

1 m/s = 3.6 km/h
1 mph = 1.609 km/h
1 knot = 1.852 km/h = 0.514 ms-1

Examples of different speeds
Main article: Orders of magnitude (speed)

* Speed of a common snail = 0.001 ms-1; 0.004 km/h; 0.002 mph (1 millimeter per second).
* A brisk walk = 1.7 ms-1; 6 km/h; 4 mph (5.5 feet per second).
* Olympic sprinters (average speed over 100 metres) = 10 ms-1; 36 km/h; 22 mph.
* Speed limit on a French autoroute = 36 ms-1; 130 km/h; 80 mph.
* Taipei 101 observatory elevator = 1010 m/min ; 16.7 ms-1 ; 60.6 km/h; 37.6 mph
* Cruising speed of a Boeing 747-8 = 290 ms-1; 1050 km/h; 650 mph; (Mach 0.85)
* The speed of sound in dry air at sea-level pressure and 20 °C (293 kelvin) is 343 ms-1 ≈ 1235 km/h ≈ 768 mph ( = Mach 1 by definition).
* Official flight airspeed record = 980 ms-1; 3,530 km/h; 2,194 mph.
* Space shuttle on re-entry = 7,800 ms-1; 28,000 km/h; 17,500 mph.
* Average orbital speed of planet Earth = 29,783 ms-1; 107,218 km/h; 66,623 mph.
* Speed of light in vacuum (symbol c) 299,792,458 ms-1 (exactly, by definition).