What it shows
A ball stores energy when it is squashed and gives most of it back as it bounces. Different balls and surfaces return different fractions of the energy, so the bounce height changes.
The science
A ball held up has stored energy because of its height. When dropped, the energy turns into movement and then, when it hits the floor, into squashing of the ball and floor. As the ball springs back, much of the energy returns as movement, but some is lost as heat and sound. The bounce is lower than the drop, and the ratio of bounce height to drop height shows how much energy returns. A hard bouncy ball on a hard floor gives the most, and a soft surface takes in the energy.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A rubber ball, a tennis ball, a ping pong ball and a cricket ball or a wooden ball
- A metre ruler fixed upright on a wall with tape
- A phone to take slow motion video
- Different floor surfaces: cement, carpet, wooden board, sand tray
- A notebook
- A calculator
- A friend
Safety
For this project: Do the drops away from windows and glass, and keep the ball low so that it does not hit anyone.
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.
Fix the metre ruler upright against the wall. Stand the camera at the side to film the ball against the ruler.
Drop the rubber ball from the 100 cm mark and film the first bounce.
Play the video frame by frame and read the highest point of the bounce against the ruler.
Repeat three times for each ball on the cement floor and take the average bounce height.
Calculate bounce height divided by drop height for each ball. This is the bounce ratio.
Repeat with the rubber ball from 50 cm, 100 cm and 150 cm and see if the ratio changes with height.
Repeat with the rubber ball on the carpet, the wooden board and the sand tray and compare.
Draw bar charts for the ratios and write which ball and which surface gave the best bounce.
What you should see
Bar charts showing that the rubber ball on cement has the highest bounce ratio and the sand or carpet has the lowest, with the ratio nearly the same for different drop heights.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the ball never bounce back to the same height?
Some energy is lost as heat and sound at each bounce.
2Which surface gives the best bounce?
A hard surface such as cement, because it does not absorb the energy.
3What is the bounce ratio?
The bounce height divided by the drop height.
4Why does a flat football bounce badly?
Without enough air inside, it does not spring back and instead absorbs the energy.
Take it further
Record the heights of three bounces in a row for the best ball and see whether the ratio stays the same.
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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