Coulomb’s Law of Friction

Abstract

In this chapter, we will only investigate the dry friction or Coulomb friction between two solid bodies. Friction between solid bodies is an extremely complicated physical phenomenon. It encompasses elastic and plastic deformations of the surface layers of the contacting bodies, interactions with wear particles, micro-fractures and the restoration of the continuity of materials, excitation of electrons and phonons, chemical reactions, and the transfer of particles from one body to the other. What is astonishing is the fact that it is possible to formulate a very simple law for dry friction. This first order approximation is sufficient for many engineering applications: The frictional force is proportional to the normal force and as good as independent from the speed. The astounding property of dry friction lies in the fact that in a first order approximation, it is dependent neither on contact area nor on roughness. This property allows us to use the notion of the coefficient of friction. The coefficient of friction, however, gives only a very rough first approximation of the quotient of frictional force to normal force.