MB Science - 13 - Bouncing High

'Try this' challenge 13: Bouncing High

You will need:

Quick start:

Some eye protection (swimming goggles, sunglasses, anything will do!)
A basketball or football, make sure it is fully inflated
A tennis ball

Genius:

Other balls - ping pong, golf etc.

DO THIS OUTSIDE! AND MAKE SURE YOU ARE WELL AWAY FROM WINDOWS, CARS ETC!

What to do:

To start, drop the tennis ball from shoulder height – make a note of how high it bounces back up.

Then do exactly the same with the basketball.

For this next bit, you'll need to put on eye protection and ask everyone to step back. Make sure you line up the centre of the balls so that the tennis ball is exactly on top of the basketball.

Give a dramatic countdown and let them go at the same time so that they drop straight downwards.

Genius challenges:

Which bounced higher when dropped by itself, the tennis ball or the basketball?
When you dropped the balls together, which bounces first: the basketball or the tennis ball?

There are lots of different ways to mix this experiment up.

Try the experiment again, but this time watch what happens to the basketball (you can’t get something for nothing!)
Try three balls stacked on top of each other: ping pong ball on tennis ball on basketball (definitely do this one outside).

If you don’t have the space outside, try our indoor version using:

A ping pong ball for the top ball
A golf ball or tennis ball for the bottom ball

Dropping balls stacked up like this (from heaviest to lightest) is called a Galilean cannon. The world record for the highest bounce from a Galilean cannon currently stands at 13.08m can you beat the record?

email your photos/ videos / explanation to trythisathome@mountbatten.hants.sch.uk to get your reward!

What is going on?

To start off with the balls are dropped separately, so let’s think about what happens to them during their journey down to the Earth (another massive round ball) and back up.

If they were dropped from shoulder height at the same time, gravity pulls down on both and so makes them get faster and faster so that they reach the Earth at the same time, with the same speed. They bounce because the collision with the Earth squashes them and as they spring back into shape they launch themselves upwards.

Each ball then travels upwards, slowing down as gravity pulls on them, until they momentarily come to a stop at the top of the bounce. For the tennis ball this is about half the height it was dropped from, and for the basketball it's a bit higher.

So what’s different when you drop the tennis ball on top of the basketball?

On the way down there is no difference. The balls fall together and both reach the same speed as before. However, the basketball bounces first. It bounces off the ground and so is moving upwards when the tennis ball bounces off it.

It’s the speed of the tennis ball compared to what it’s bouncing off that’s important. This is called the relative speed. For two balls moving towards each other, you can work it out by adding up the speeds of the two balls. The tennis ball bounces higher because the relative speed when it bounces off the basketball is almost double that for when it hit the ground