A tennis ball will have zero kinetic energy when it is at rest, meaning that its velocity is zero. When the student drops the ball, it accelerates downward due to gravity and gains kinetic energy as it falls. When it reaches its lowest point before it starts to bounce back up, it will momentarily come to a stop. At that point, its velocity is zero, and therefore, its kinetic energy is also zero.
To summarize, the ball has zero kinetic energy at two points in this scenario:
- Just before it is dropped (when it is held and not moving).
- At the lowest point of the bounce just before it starts to rise again (when it comes to a momentary stop).
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