# Explaining elastic and inelastic collisions

Demonstration

In elastic collisions the kinetic energy (KE) is conserved. In inelastic collisions the KE 'goes somewhere'. But where? This demonstration helps students picture a possible route for loss of KE.

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.

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

a 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.

b 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

1 You can explain these observations as follows:

• Empty bucket. The ball has little to share its kinetic energy (KE) with (apart from the bucket), so recovers most of it and bounces back to almost the same height.

• Partly full bucket. The ball loses some of its KE by colliding with one or two other balls of similar mass - it is easy to share KE in this fashion, so it bounces out much lower.

• Full bucket. There are now many balls which rapidly share the KE, so the original ball makes many quick collisions and does not bounce out. The other balls also share the KE so no single other ball is knocked out.

2 In an inelastic collision, KE is transferred to other objects much as in the last example - it is rapidly spread out and randomized. This is thermal energy - random motion of particles.

Note: this particular example does not explain deformation energy, but it does try to help students visualize the difference between KE of a single particle and KE 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.

Page last updated on 05 October 2011