Re: Why does a ping pong ball, released just under water, shoot up quickly?

Hi Kathy,

The motion of the ping-pong ball is a matter of energy. In the water, the ball is buoyant, i.e. it wants to rise upward. Oddly enough, this is due to gravity, which you would ordinarily think would make everything "fall" down.

The difference with the ping-pong ball in water is that, as it moves upwards, water moves downwards to replace it. The volume of water that replaces the ping-pong ball has more mass, and so effectively, as the ping- pong ball rises, mass is actually moving downwards in the net. This is an energetically favorable situation, and so the ball continues to rise. It cannot accelerate indefinitely as it rises, because of drag, which results from the energy that the ball has to give to the water in order to displace it as the ball moves upwards. The drag increases with the speed of the ball, and so the ball will reach a terminal velocity, like a skydiver.

The situation reverses itself, rather suddenly, when the ball reaches the surface of the water. The ping-pong ball has a higher average density than air, and so it is "negatively buoyant" (it wants to sink). The ball is therefore driven quickly to its "natural" state (at the risk of sounding a bit Aristotelian), of floating on top of the water.