Wednesday, March 11, 2009

☻stat!c fR!ction 2

Static Friction

PIRA #: 1K20.35
setup time: 1 minute

Description:

An inclined plane is used to measure the coeffiecient of static friction between it and a wooden block.
Theory:
If an object is at rest on a surface and you push against it and it doesn't move, is there friction on the object?

The answer is "yes", but this kind of friction is different from kinetic friction. Whereas kinetic friction acts to resist the motion of an object sliding across a surface, static friction is the force which keeps a motionless object from being pushed or pulled across a surface.

If a wooden block is at rest on a horizontal wooden surface, it is acted upon only by the normal force and the gravitational force.

If the surface is inclined by a small angle, θ, a component of the gravitational force acts downward along the surface of the board. The magnitude of this component is mg sinθ. If the block doesn't slide, then it is acted on by the static frictional force, fs, which exactly balances the mg sinθ component of the object's weight.


If the inclination of the surface is increased further, the static friction reaches a maximum strength. Unlike kinetic friction, which is roughly constant at low speeds, static friction varies to resist other forces on the block. The formula for static friction, therefore, gives only the maximum possible value of the static frictional force. The formula is as follows:

If the surface is inclined so much that the mg sinθ component of the object's weight exceeds fs max, then static friction is overcome, and the block begins to slide. At the angle where the block is just on the verge of slipping, fs max is equal to mg sinθ., and Newton's laws give us that

So the coeffiecient of static friction is equal to the tangent of the angle of inclination where the block is just on the verge of slipping.

Apparatus:

* Wooden block
* Incline plane equipment

Procedure:
Place the wooden block on the plane surface and slowly incline the plane in small increments. As you are inclining the plane, gently hold the wooden block with your finger tips. This will ensure that the block will not accidentally be shaken and slide prematurely. At a certain inclination, the static friction will be overcome by the weight of the wooden block and the block will slide down the incline. The tangent of this angle is equal to the coefficient of static friction for this block and this plane.

Video:
high resolution video

3.6 MB


lower resolution video

2.0 MB



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