Kinetic energy is the energy that an object has due to its motion. A tennis ball has zero kinetic energy when it is at rest, which occurs at the highest point of its bounce.
When the student drops the ball, it accelerates downward due to gravity, gaining kinetic energy as it falls. When the ball hits the ground, it momentarily comes to a stop (its velocity is zero) before changing direction and bouncing back up. At the peak of its bounce (the highest point it reaches after bouncing), the ball again comes to a momentary stop before it starts to fall back down.
Thus, the ball has zero kinetic energy:
- Just before it hits the ground (when it briefly stops just after dropping or before bouncing)
- At the peak of its bounce.
In these moments, all of the ball's energy is potential energy, and it possesses no kinetic energy.
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