Impact behavior of a superball

When a superball is incident with backspin on a horizontal surface, at an angle near the normal, it bounces backwards and with a reversal in the spin direction. The effect is less noticeable with other balls, which may bounce with reduced spin and without a reversal in the spin direction. The outcome depends on the friction force acting on the ball. Measurements with several different rubber balls are presented showing that the time history of the friction force, the resulting tangential impulse, and the resulting tangential coefficient of restitution all depend on both the coefficient of sliding friction and the ratio of the tangential vibration period to the impact duration of the ball. Grip and slip phases of the bounce are identified visually by allowing the ball to impact on chalk lines drawn on a blackboard, and by video-recording the bounce of a rectangular block of rubber. (C) 2015 American Association of Physics Teachers.