The force that resists the motion of one surface relative to another with which it is in contact. For a body resting on a horizontal surface there is a normal contact force, R, between the body and surface, acting perpendicularly to the surface. If a horizontal force B is applied to the body with the intention of moving it to the right, there will be an equal horizontal friction force, F, to the left, resisting the motion (see illustration). If B is increased until the body just moves, the value of F will also increase until it reaches the limiting frictional force (FL), which is the maximum value of F. FL is then equal to µsR, where µs is the coefficient of static friction , the value of which depends on the nature of the surfaces. Once the body is moving with constant velocity, the value of F falls to a value Fk, which is equal to µkR, where µk is the coefficient of kinetic friction . Both µs and µk are independent of the surface area of the body unless this is very small and µk is almost independent of the relative velocity of the body and surface.
The cause of friction is that surfaces, however smooth they may look to the eye, on the microscopic scale have many humps and crests. Therefore the actual area of contact is very small indeed, and the consequent very high pressure leads to local pressure welding of the surfaces. In motion the welds are broken and remade continually. See also rolling friction.
Friction
- pH
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- phases of the moon
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