This freaky friction game will get sport science on the run.
WARNING: This activity uses a rubber ball which could cause an eye injury. Don't throw too hard. Keep eyes out of the balls path.
You will need
a flat board, this can be anything, a bit of Perspex, plywood, an upside down table, even the lid of a large container
a super ball – a bouncy ball that is made from rubber
a cup of water
a table
three players
What to do
The game goes like this: one player stands in the middle and holds the board about 50 cm above the table. The other two players stand on either end of the board and attempt to throw the ball to each other.
The ball leaves your hand and hits the table, it then bounces up and hits the board (think upside down table tennis), the aim is to then make it bounce back down to the table and through to the other player.
Let the game begin! Attempt to bounce the ball as described above and give both players a try. Throw gently until you get the hang of it. What happens?
Now dunk the ball in water. The water will make the ball all slippery which is good and what you want.
Now try the game again – does it work this time?
If your ball has patterns on it, observe closely which way the ball is spinning when it is wet and dry and repeat a few of each. Drawing a line around the ball can help your spinning observations. Can you work out what is going on?
What's happening?
Friction is to blame in this game. It causes the ball to bounce back out the way it came in. How? Friction makes the ball grip the table when it first hits, and this makes the ball spin. The ball then hits the board and due to the spin, it bounces back to the thrower.
Friction is a force that happens when surfaces are in contact. It is defined as the resistance of any two things sliding past each other. Most surfaces are rough if you look close enough and when two surfaces rub together, the force of friction comes into effect.
When the ball is wet, there is not as much friction, as the water acts as a lubricant. When the friction is reduced, the ball can't grip as well and hence doesn't start to spin, so it bounces smoothly through.
Applications
The force of friction is very useful in sport – you need it to catch a basketball or a cricket ball. It is the friction between you and the ball that allows you to grip, kick and pass the ball. Imagine trying to kick a slippery slimed up ball – now you know why ball sports are so much trickier in the rain.
Friction can also be very handy in sports like cricket and ten pin or lawn bowling. Warney's legendary spin bowling uses just the same physics of friction to change the ball's direction and you will see the ball spinning like mad as it leaves his had.
Sports like snowboarding use the force of friction to help you tear down mountains. As gravity pulls you down the slopes, friction helps you steer and friction slows you down when you want to stop. The water lubricating the ball plays a similar role in snowboarding – as your board presses on the snow it causes a very small amount to change into water, lubricating the underside of the board.
There is friction acting between the board and the snow but there is also friction between you and the air. Even if you were in a skin tight Lycra body suit, there will be some frictional force (or drag) from the air. The faster the snowboarder goes, the greater the frictional force from air, or drag.
