Why a squash ball needs to warm up to bounce properly

Partly settled. Some of it is understood, some is still argued about.

Squash players have long known a cold ball from a cool equipment bag barely bounces and must be 'warmed up' by vigorous hitting before a match, but the underlying materials-physics explanation is less commonly known outside sports engineering. A 2011 American Journal of Physics study used drop-bounce and compression tests to characterize squash balls across a range of temperatures, finding the coefficient of restitution rises sharply with rubber temperature due to the material's viscoelastic behavior and time-temperature superposition properties characteristic of polymers near a glass transition, while internal air pressure changes played a comparatively minor role. This gave squash's pregame ritual a precise physical grounding in rubber viscoelasticity rather than folk intuition about air pressure, though the draft's original source hint flagged this as needing verification since coverage of the exact topic is less centralized than for more famous physics puzzles.

The ritual thumping every squash player does before a match to 'warm up the ball' is really coaxing a polymer through part of its glass transition, one bounce at a time.

Explanations that circulated

• The common assumption that warming up mainly increases the ball's internal air pressure (like inflating a tire) — the AJP study's compression tests found internal air pressure contributes comparatively little to energy loss, with the rubber's own temperature-dependent viscoelastic (glass-transition-like) stiffening the dominant effect

References

• The dynamic behavior of squash balls, A. Lewis, S. Arnold, I. Griffiths (author names as reported in secondary sources; verify spelling), American Journal of Physics, 2011