Science of spin and bouncy balls

15 October 2006


This week Derek is with Professor Hugh Hunt of Cambridge University and student helpers James and Martin from the Norwich School. They're going to be finding out about the science of bouncing balls.

To do this experiment, you will need:

Super bouncy ball A little bit of oil or vaseline Kitchen counter that meets a vertical tiled wall

How to do the experiment:

1 - Take the bouncy ball and roll it gently across the work surface towards the wall. Watch to see how far it bounces back.

2 - Put a little oil or vaseline on the tiles where the ball hits the wall.

3 - Roll the ball across the work surface again and watch how far it bounces back.

What's going on?

The point of this experiment is not only to look at how far the ball bounces back, but to think about how the spin direction changes. If you think of the ball rolling clockwise when it moves towards the wall, then on the way back it must be rolling anticlockwise. This change in spin direction requires energy, but where is the energy coming from?

When rolling the ball against a dry wall, you may have noticed that it started to bounce on the way back. It is this bounce that helps the ball to change its spin direction, and it's the wall that's providing the kick.

Taking a closer look, you will see that as the ball approaches the wall, it tries to continue rolling upwards in the direction it was going (ie: clockwise). However, gravity means that it can't do that for very long, and so it has to fall back down again. This makes the ball start to bounce and roll the other way (to see a graphical simulation of this effect, go to Hugh Hunt's website).

So what happens when there is oil on the wall? In contrast to when the wall was nice and dry, the ball can no longer grip and roll upwards. This prevents the ball from bouncing and can no longer use the wall to change its spin direction.

The result is that the ball starts rolling back to us spinning the wrong way (ie: still in the clockwise direction). The only way it can start to spin anticlockwise is by relying on friction between the ball and the kitchen counter. However by the time it's done so, the ball has lost all of its energy and comes to a complete stop.

This is why the ball doesn't roll back as far when there is oil on the wall. The oil reduces the friction, stops the ball getting a kick up the wall, and leaves the ball still rolling forwards even though it's moving backwards.

So can we see this effect anywhere else? You might expect to see balls bouncing in this way when they roll towards the cushion in a game of snooker. In fact, you won't see this at all because the cushions are specially designed so that the ball won't jump when it bounces back. The cushions are angled so that the point of contact between the ball and the cushion is not exactly half way up the ball (as when it hits a vertical wall), but higher up on the ball's surface.


Add a comment