Try this: Bounce back

This week, we will look at a bit of sport-related science.

You will need

• A bouncy ball, with rubber on the outside. A superball or high-bounce ball is ideal.
• A table.
• A tray or some other flat object. I used a folder.
• A glass of water.
• Another person.

What to do

1. Have a person stand at each end of the table.
2. Hold the tray about 30cm above the table.
3. Give the ball to one of the people at the end of the table.
4. Ask them to throw the ball so it bounces off the table, hits the tray, hits the table again and then bounces out the other side (so it will go in a W-shaped path).
   1. Make sure the person throwing only throws it gently.
   2. You may need to adjust the height of the tray to help them.
5. Instead of going through, the ball will bounce back in the direction it came from.
6. Once you've had a laugh, dip the ball in water.
7. The wet ball will go through.

What's happening?

There are two things that make the ball bounce backwards: Friction and Spin.

Friction is a force that occurs whenever two objects are pressed together. Dynamic friction is the force you feel when two objects rub against each other. If you try to slide the ball along the table, without rolling it, the force you feel pushing against you is friction. Rubber tends produce a lot of friction when it rubs against things, which is why we use rubber in tyres and shoes.

Rubber balls are very elastic, which means that when they hit something they bounce back in the opposite direction. If you drop the ball straight down onto the table, it will bounce straight up again. If you give the ball a bit of sideways spin when you drop it, it will bounce off in the direction of the spin. When it hit the table, the rubber at the bottom of the ball was moving sideways. Because of friction, the rubber grips the table and bounces back in the opposite direction. This pushes the ball off to the side and reverses the direction it is spinning. (There is a diagram of this on the web site.)

When a spinning ball bounces, it reverses its spin and bounces off to the side.

When you threw the ball to make it bounce up and hit the tray, this is what happened:

1. The first time the ball first bounces off the table, the ball kept moving forward, but friction between the table and the ball at the point where they touched made the ball start to spin with the top side of the ball going forward and the bottom going backwards.
2. When the upper side of the ball hit the tray, the bit where it touched was moving forwards, so when it bounced it was pushed backwards. Just like when you dropped a spinning ball on the table, it also reverses the direction it spins.
3. When the ball hits the table the second time, it was still spinning. However, because of the direction it is spinning, when it hits the table it is pushed forward, back to the person that threw it.

When rubber is wet, it becomes slippery, so there is very little friction between the ball and the table or tray. When you throw the wet ball, it barely spins at all when it hits the table and even if it did spin, it doesn't grip the tray enough to bounce backwards.

Applications

• Spin is important in almost all ball sports, including soccer, cricket and tennis. By putting a spin on a ball when it is kicked or thrown, a player can make it change direction when it bounces (as well as make a curved path in the air).
• Friction is also important in many sports. Sports shoes and tyres on bicycles and cars rely on friction to work. Just like your rubber ball, tyre rubber becomes slippery when wet. The tread in road tyres is designed to funnel water out of the area where the tyres touch the ground, to stop them from sliding