Oblique angle collisions of two pendulum balls

Abstract

Measurements are presented on oblique angle collisions between two identical balls, each suspended as pendulum bobs. If one ball is at rest and is struck by the other ball incident without spin at an oblique angle, then both balls spin in the same direction after the collision. The results can be explained in terms of friction between the balls. At glancing angles, the balls slide throughout the collision. At other angles of incidence, the balls initially slide, then grip, and then slide again when the balls release their grip. The collision can be described in terms of two different coefficients of restitution: one involving the normal incident and post-collision speeds and the other involving the tangential speeds of the contact points before and after the collision. The coefficient of restitution and the collision time vary with the collision speed but remain relatively constant if the speed changes by a factor less than about two.