What it shows
The more air pressure inside a ball, the harder it is and the higher it bounces. A ball that is too soft loses a lot of energy because it squashes more on impact.
The science
A higher pressure makes the ball skin stiffer and stops it from squashing as much. A stiff ball returns more of its energy and bounces higher, while a soft one wastes energy as heat in the rubber. The pressure is limited by the strength of the skin, so each ball has a maximum.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A football or volleyball with a valve
- A hand pump
- A pressure gauge for ball valves
- A tape fixed on a wall
- A phone for slow-motion video
- A notebook
Safety
For this project: Do not over-pump past the maker’s printed limit on the ball. An adult must handle the pump and gauge, and stay clear of the ball when bouncing.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Read the recommended pressure range printed near the valve of the ball.
Deflate the ball and pump it up to the lowest value in the range with a gauge.
Hold the ball so its bottom is at 1.5 m on the wall tape and drop it onto a flat concrete floor.
Film the bounce and read the highest point of the first rebound.
Repeat three times and take an average.
Pump the ball up in equal steps up to the upper limit and repeat the drops each time.
Plot a graph of rebound height against pressure.
Describe the trend, and explain why the line eventually flattens.
What you should see
A rising curve of rebound height with pressure that levels off. A very soft ball bounces poorly, and the rise slows at the top of the range.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is pressure?
Force per unit area.
2Why does a soft ball bounce lower?
It squashes more and loses more energy as heat.
3Why must you not exceed the limit?
The skin and seams could burst or be damaged.
4Why drop from the same height each time?
To keep the starting energy equal.
Take it further
Compare the bounce at the same pressure when the ball is cold from a fridge and when it is warm.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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