Why does a trampoline push a jumper back upward?

At the bottom of a trampoline bounce, a jumper can be still for an instant and already beginning to speed upward. The stretched trampoline supplies the upward push. Its flexible bed and elastic supports have changed shape as the descending jumper pressed them down, storing some of that movement energy.

The stretched parts pull toward their original shapes. Through the bed, they push upward on the jumper. Near the lowest point, this upward force can be greater than the jumper’s downward weight. The difference changes the jumper’s motion from downward toward upward.

This moment separates two ideas that are easy to confuse: speed and acceleration. Speed tells how fast something is moving now. Acceleration tells how its motion is changing. At the lowest point the speed can briefly be zero while the acceleration points upward, just as a journey can pause while its direction reverses.

As the trampoline returns, stored energy becomes movement again. Some also spreads into heat, sound and motion of other parts. The trampoline does not create a new supply on every bounce; a jumper’s muscles can add energy by moving at the right time. What looks like a springy surface giving energy away is really a surface storing energy and then returning part of it.