From Smithells Metals Reference Book, Eighth Edition

The friction and wear characteristics of materials are not intrinsic properties but, rather, depend on a large number of variables including the physical, chemical and mechanical properties of the material and surfaces and the environment.

25.1 Friction

25.1.1 Friction of Unlubricated Surfaces


Friction is the resistance to motion when two bodies in contact slide on one another. The frictional force F is the force required to initiate or maintain motion. If W is the normal reaction of one body on the other, the coefficient of friction is defined as = F/W.


If the force to initiate motion of one of the bodies is F s and the force to maintain its motion at a given speed is F k, there is a corresponding coefficient of static friction s, = F s /W and a coefficient of kinetic friction k = F k/W. In some cases these coefficients are approximately equal; in most cases s, > k and there is a tendency for intermittent or 'stick-slip' motion to occur.


The two classic laws of friction, which are valid over a wide range of experimental conditions, state that:

  1. The frictional force F between solid bodies is proportional to the normal force between the surfaces, i.e. is independent of W.

  2. The frictional force F is independent of the apparent...

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