F) that must be applied to an object to initiate and maintain relative motion is proportional to the applied load (
L). The proportionality constant is the coefﬁcient of friction (
Virtually in all dry sliding contacts we observe that the frictional force required to initiate motion is more than that needed to maintain the surfaces in the subsequent relative sliding: thus there are two values reported for the coefﬁcient of friction. The
static coefﬁcient of friction is used in reference to the initial movement of the object from the rest position. In this case, the
kinetic coefﬁcient of friction is used for two surfaces in relative motion. This feature, together with the inevitable natural elasticity of any mechanical system, can often lead to the troublesome phenomenon of stick-slip motion (the displacement of surface materials with time). Displacement increases linearly with time during periods of sticking; when slipping occurs, the deformed surface materials are released. Representative of dry static and kinetic coefﬁcients of friction for various material pairs are found in tribology and physics references;11 see Table 3.1.