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Bounce back

By Jasmine, 17 January 2013 Activity

Six different coloured sports balls.

Make your sports skills shine! Find out more about the physics of bouncing balls!
Image: Thinkstock

Challenge your friend to bounce a ball across a table. With this one simple trick, you can get it to bounce back!

Written by Justin McGuire

You will need

  • Bouncy ball, with rubber on the outside
  • Table
  • Tray or other flat object
  • Glass of water
  • Another person

What to do

  1. Have a person stand at each end of the table.
  2. Have one person hold the tray about 30 cm above the table.
  3. Give the ball to the other person. 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). Make sure the person throwing only throws it gently. You may need to adjust the height of the tray to help them.
  4. Instead of going through, the ball will bounce back in the direction it came from.
  5. Once you’ve had a laugh, dip the ball in water. 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 balls are very elastic, which means that when they hit something they bounce back in the opposite direction. Try dropping the ball straight down onto the table. The ball will bounce straight up again. Then, if you give the ball a bit of sideways spin when you drop it, it will bounce off in the direction of the spin.

In this activity, the first time the ball bounces off the table, the ball keeps moving forward, but friction between the table and the ball at the point where they touch makes the ball start to spin with the top side of the ball going forward and the bottom going backwards.

When the upper side of the ball hits the tray, the bit where it touches is moving forwards, so when it bounces it is pushed backwards. Just like when you drop a spinning ball on the table, it reverses the direction in which it spins.

When the ball hits the table a second time, it is 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.

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.

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