Science Experiments

The Spin of a Bouncing Ball

Tue, 17th Jul 2012

Listen Now    Download as mp3 Part 1,2 from the show Better to blow up an Earth-bound Asteroid?

What you Need

A super bouncy rubber ball

A low table which is smooth underneath (with no structure getting in the way) standing on a solid non-slippery floor (carpet won't work).  Otherwise a wooden board supported 20-30cm over a table will work well.

What to do

Take the ball and throw it so that it bounces off the floor, then onto the bottom of the table.

Do this gently, somewhere there aren't lots of delicate objects which could be easily broken.

Does the ball do what you would expect?

What may happen

You should find that the ball bounces back at you out from under the table.

Why does it happen?

if you did the same experiment with a slippery ball, or on a slippery surface, you will find it will behave as you would expect, bouncing under the table and coming out the other side.

Simple path with no friction

When the ball bounces there is a large reaction force up from the table which pushes it back up and making it bounce. No other forces  act on it so it slips across the table, and keeps going at the same speed horizontally.

Simple bounce with no friction

However if you use a super bouncy rubber ball, it grips the surface where it hits. This means that there must be a large frictional force on the ball which starts it spinning, in fact because the ball is elastic its spin bounces and it starts to spin the other way.  This horizontal force causes the bounce to be more vertical than normal.

Friction collision with floor

The ball keeps spinning as it moves upwards and then hits the underside of the table, it isn't moving horizontally very fast now, but it is spinning rapidly, and in the collision the friction causes this spin to reverse, and actually pushes the ball in the opposite direction.

Fricitonal collision with table

The ball then hits the floor and the spin acts to speed it up horizontally and it come out from under the table.

Bounce path with friction

To find out more fascinating ways that spin can do entirely unpredictable things visit Professor Hugh Hunt's website


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