Friction of a pimpled rugby ball surface: Force and velocity weakening and strengthening of the coefficient of friction

Friction of rubber is known to behave non-linearly, depending on contact pressure and sliding speed. The surface of a rugby ball made of natural rubber with regular pimples was investigated, and the coefficient of friction between finger skin and rubber surface was determined at normal forces of 0.4-40 N and sliding speeds of 0.001-3.2 m/s. The kinetic coefficient of friction asymptoted to the static coefficient at small speeds. The coefficient of friction exhibited force strengthening and weakening, as well as velocity strengthening and weakening. The maximal coefficient of friction amounted to 3.55 at a normal force of 1 N and a sliding speed of 70 mm/s. When throwing a ball, the coefficient of friction is static and in the force weakening regime at the beginning of the acceleration phase, then moves to the velocity strengthening regimes once finger sliding sets in and finally crosses over to the force strengthening and velocity weakening regimes before the fingers leave contact with the ball.