Teacher Demonstrations
A bounce plus a bounce?
Ask your students to do some simple bounce arithmetic
and they’ll spring to attention
Materials: Large ball (basket ball works best) / Small ball (a mini-soccer ball works best)
Instructions:
y Drop the larger ball from shoulder height and ask the class to rate
the height of the bounce from zero to ten (zero is no bounce at all
and ten is a bounce up to the shoulder)
y Repeat the bounce rating for the smaller ball.
y Now hold the two balls so that the smaller one is resting and
centred on top of the larger one. Ask the class how high they think
the smaller ball will bounce when you let them go simultaneously.
You could suggest that it might be the sum of the ratings they
gave the balls individually.
y Drop the two balls drop simultaneously. The smaller ball will shoot
much higher than the sum of the two ratings. In fact, it may easily
hit the ceiling!
y Repeat the demonstration a few times and ask the students to
observe what is happening to the larger ball. They should notice
that it hardly bounces at all now. Ask if anybody would like to
attempt an explanation being careful to praise any attempt as this
is a demonstration of a reasonably complex collision.
Explanation:
The physics of this demonstration is actually quite complicated – a mathematical description would be
beyond the scope of most first year university physics courses. But a conceptual explanation will be easily
understood your students.
You would have noticed that the larger ball hardly bounces at all in this demonstration. On its own, it
bounces a certain height and all this energy is now being transferred to the smaller ball during the
collision. Both the kinetic energy and momentum of a moving object depend on its mass (weight). A
heavier object has more kinetic energy and more momentum than a lighter object moving at the same
speed. So the energy received by the smaller ball makes it go faster because it weighs less.
Safety note:
Switch off ceiling fans before performing this demonstration. Try it on your own a few times first. The
smaller ball has a tendency to fly in any direction if not properly centred on the larger ball when it
collides with the ground. Put away any breakable items from the area where you are performing the
demonstration and have students sitting about 2 metres away.
© Ruben Meerman 2004