Episode 13: Bouncing High
For the final episode in our series, we have something special. An experiment that’s sure to put a spring in your step – but hopefully not a hole in your window!
Before you start:
These experiments have not been specifically safety tested for home use but we believe them to be safe if the instructions are followed. Adult supervision or direction is recommended as appropriate. All experiments are carried out at your own risk.
What you’ll need:
- Some eye protection (swimming goggles, sunglasses, anything will do!)
- A basketball or football, make sure it is fully inflated
- A tennis ball
What to do:
In this activity you’ll be using your basketball to launch the tennis ball high into the air. To minimise any breakages in your home we recommend trying the trick outside (or try our suggestions for a more indoor friendly version further down).
- 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.
Be astounded by how high the tennis ball flies!
What to talk about:
- 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?
What’s 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.
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
Did you know?
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!
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