ABSTRACT
The friction coef cient (µ) is a convenient way to characterize the resistance to relative motion between surfaces, but it is not a material property, nor is it a physical constant like the speed of light in a vacuum or Avogadro’s number or the elementary charge on the electron. For a given pair of materials, µ may, on average, remain relatively constant over a range of contact conditions, but transitions in friction sometimes occur and one would be ill-advised to extrapolate (or even sometimes to interpolate) the value of µ far beyond the conditions under which it was originally measured. Therefore, it is of both fundamental and practical importance to understand the effects of variables such as sliding speed, normal force (contact pressure), temperature, environment, and material properties on frictional behavior.
