Coulomb's law for rolling friction

Measurements are presented on the coefficient of rolling friction for steel balls rolling on hard surfaces. Two simple techniques are described, both suitable for use in a student laboratory, and both capable of measuring friction coefficients as small as 0.0001. The coefficient of rolling friction depends strongly on ball radius, an effect first observed by Coulomb in 1785. In this work, the dependence on ball radius is found to be similar to that observed by Tabor in 1955 using steel balls on rubber and on soft metal surfaces. However, it is found that rolling friction on a hard surface is due primarily to surface roughness rather than the hysteresis losses commonly associated with soft balls or soft surfaces. It is also found that the coefficient of rolling friction is approximately proportional to rolling speed. (C) 2016 American Association of Physics Teachers.