Balloon Over Glass


Take a balloon, carefully cut the neck off using scissors, then stretch the rubber over a drinking glass. Let a marble or steel ball bearing bounce and observe. What happens and why?



When things bounce, they collide inelastically with a surface. The balloon-glass-air system absorbs very little energy from the ball on each collision, so the ball will bounce up and down for a very long time. A balloon on glass has a very high coefficient of restitution.

Thoughts and Opinions

Air is a very key component of what makes this toy work.

Air is a compressible fluid, so it can change its pressure, volume, and even temperature to compensate changes in either one of the three. The time it takes for a collision with the balloon surface is very small, so these changes occur approximately adiabatically. In other words, in the very tiny time the ball is squishing on the balloon, the air inside the glass does not have enough time to lose energy to the environment. The result is a push just big enough to almost return all of the energy back to the ball for another bounce. If no energy was lost at all by the ball, it would bounce to the same height forever.

This is a surprising experiment that is also very affordable and very simple to do. If you are having trouble keeping the ball on the balloon, let a little air out so that the balloon forms a concave surface. On each bounce, the ball will tend to move more towards the middle.

I want to share this exercise because it is fun and it is beautiful. For me, it helps my imagination visualize what would happen if a ball could bounce forever.

Forever, eternity, and infinity are allowed in this place.