Why does a spinning superball bounce backward off the floor?
Why does a Superball thrown with backspin straight down sometimes bounce back toward the thrower instead of continuing forward, and why does its spin direction reverse after the bounce?
History and references
Settled in 1969 by R. Garwin (1969).
A Superball is unusual in combining very high elasticity (nearly perfect energy return) with high surface friction, so when it strikes the floor with sufficient spin, the tangential (frictional) impulse during the brief but non-negligible contact time is large enough to not just stop the spin but reverse it, while simultaneously reversing the ball's horizontal translational motion -- effectively 'rolling' off the collision the way a bouncing ball rolls without slipping. Garwin's 1969 American Journal of Physics paper derived the full kinematic conditions (in terms of the ball's moment of inertia, coefficient of friction, and contact geometry) under which this counter-intuitive backward bounce occurs, and it has become a standard demonstration in introductory mechanics.
A cheap rubber toy ball demonstrates that friction, not just elasticity, can completely reverse both an object's spin and its direction of travel.
Explanations that circulated
- The reversal is caused by the ball's elasticity alone, with no role for friction at the contact point
- Backward bouncing only happens due to imperfections or off-center impacts, not as a general predictable effect
References
- Kinematics of an ultraelastic rough ball, R. Garwin, American Journal of Physics, 1969
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