- large ball that bounces (e.g. football or basketball)
- small ball that bounces (approx 10cm in diameter)
- Hold out your hand at shoulder height. Ask your audience to imagine a scale from 0 to 10, with 0 being the ground and 10 being the height of your hand.
- Pick up the large ball and hold it out at shoulder height. Tell your audience that you are going to drop the ball, and that they should judge (on the imaginary scale) how high it bounces.
- Drop the ball and ask the audience how high it went (get a rough value).
- Pick up the small ball and repeat the demonstration.
- Hold the small ball on top of the large ball at shoulder height and ask the audience what they think will happen to each of the balls if they are dropped.
- Drop the balls. The small one will shoot off much higher than the sum of the original bounces put together. Repeat the demonstration, asking the audience to closely watch the larger ball. You will see that it hardly bounces at all.
How Does it Work?
This experiment is all about conservation of energy and momentum. When the balls are dropped together most of the momentum from BOTH balls is transferred to the small ball. Both the kinetic energy and the momentum of any moving object depends on its mass. If the smaller ball receives all the kinetic energy and momentum from the larger ball it will bounce much higher than the original larger ball because it is so much lighter. Add to that the original energy and momentum in the smaller ball and you get a bounce that is much greater than the sum of the two original bounces. There are also complications due to the materials used to make the balls ('bouncy' balls go wild!). This experiment can also be used as a good demonstration of Chaos effects – small changes in the initial conditions (e.g. exactly how the two balls are held above one another) can cause large differences in the end result.
Tips for Success
For indoor spaces use a small ball that isn't too bouncy or it will go crazy and could potentially do damage. You could try using a long ruler to help the audience judge how high the balls bounce (some audiences have difficulty with that part and don't get so involved).
This is a good visual trick that can draw in audiences – especially in outside venues. In outdoor venues it can be a lot of fun to use a bouncy ball as the smaller ball – although you may have to chase after it quite a bit!