Use bouncing balls to demonstrate collisions and energy transfer with this simple physics experiment.

## Safety

This activity involves balls flying around in random directions. Do it outside or in an open space. Do not use hard balls or any ball smaller than a tennis ball.

## You will need

• tennis ball or other similar small ball.

## What to do

1. See the energy transfer when you drop a tennis ball and basketball as shown above

Using your body as a point of measure, drop each ball from shoulder height and take note of where it bounces back up to (knees, waist, chest, etc).
2. Hold up two balls with the smaller, lighter ball balanced on top of the other.
3. Drop the balls at exactly the same time, making sure they are perfectly aligned.

## What's happening

When the balls hit the ground, the small ball on top will, and you might need to practice this, shoot off very high. The larger ball on the bottom falls dead when it hits the ground, as though it is flat.

Energy transfer during a collision is a part of everyday life and one of the most interesting examples is sport.

The bottom heavier ball has transferred most of its energy to the smaller, lighter ball. The energy has been transferred to the small ball during the collision. Because the energy has left the larger ball it now only has a small amount of energy, which results in a small bounce.

When you lift a ball up you are giving it energy. In this case you are giving it potential energy, which means the ball has the potential to do some work.

When you drop the ball, it gains moving (kinetic) energy as it loses its potential energy. When a ball is dropped, it will not bounce up to the original height from which it has fallen, as you discovered in step 1.

This is because some of the energy has been transformed to other energies when the ball collides with the ground. If all of the moving energy was kept in the ball, the ball would bounce back to the original height that it was dropped.

Some energy has obviously been transformed into another form during the collision. No energy is lost, as energy cannot just disappear, but merely changed from one form to another:

• Some potential energy has gone into making the sound of the ball hitting the ground.
• Some energy has gone into making the floor vibrate. You can discover this by asking your observer to place their hand next to where the ball is bouncing (you have to make sure you are very accurate when you drop the ball, so you don't hit your friend).
• Some energy has changed into heat energy. If you continue to bounce the ball in the same spot the ball will also start to heat up. If you play squash or know someone that does, ask them if the ball feels warmer after a big game - it will.
• Some kinetic energy has gone into deforming the ball slightly, but being bouncy balls they spring back to their normal shape. In other collisions, like a car crash, enormous amounts of energy are used up deforming objects.

## Applications

Energy transfer during a collision is a part of everyday life and one of the most interesting examples is sport. Think of a football tackle between a big and small player, it is very similar to the experiment's big and small balls.

When a ball is kicked, moving energy in the player's leg is transferred to the ball. Other games - from snooker to ping pong - rely on the physics of energy transfer.