Explaining elastic and inelastic collisions

Demonstration

In elastic collisions the energy stored kinetically is conserved. In inelastic collisions it is not. But where does it go? This demonstration helps students picture a possible route for the apparent loss of energy.

Apparatus and Materials

• very bouncy balls, about 50
• bucket

Health & Safety and Technical Notes

Balls may bounce around but are not much of a danger unless trod on. Ensure that students stand back from the demonstration area.

Read our standard health & safety guidance

There should be sufficient balls to fill the bucket to about 3 layers deep, and the bucket should be deep enough that, when all the other balls are in and one ball is dropped in, the dropped ball doesn't bounce out.

Procedure

1. Place the empty bucket on the floor in front of you. Drop a ball into it from about chest high. It should bounce back up to you, out of the bucket.
2. Now pour some of the balls into the bucket - enough to make a single loose layer. Again drop the ball. It should bounce back much lower than before. Possibly it might knock another ball out. Finally put all the balls bar one into the bucket. Again drop the ball. It should stop more or less dead and not bounce out again.

Teaching Notes

You can explain these observations as follows:
• Empty bucket. The ball makes a single collision (with the bucket). Very little energy is now stored thermally in the bucket. The ball bounces back to almost the same height.
• Partly full bucket. The ball makes collisions with one or two other balls of similar mass. Some energy is now stored thermally in the balls, so it bounces out much lower.
• Full bucket. The ball makes multiple collisions. Each ball stores a little energy thermally after the collision, so the original ball does not bounce out.
• In an inelastic collision, energy stored kinetically is transferred to other objects much as in the last example. It is rapidly 'spread out' so that it is stored in many places. This is energy stored thermally in the random motion of particles.

Note: this particular example does not explain the role of deformation, but it does try to help students visualize the difference between the energy stored kinetically of a single particle and energy stored thermally which has become randomized.

This experiment was submitted by Ken Zetie, Head of Physics at St Paul's School in West London. He is on the editorial board of Physics Education and regularly contributes to Physics Review.