What it shows
How high a ball bounces depends on both the ball and the surface. Some of the ball’s energy is lost as sound, heat and a dent in the surface at each bounce, which explains why pitches matter in cricket.
The science
A ball dropped from height h has gravitational energy mgh. On hitting the ground, it is squeezed and then springs back, but part of its energy is lost. The ratio of rebound height to drop height measures how elastic the collision was. Soft surfaces absorb more energy, so they give a lower ratio.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A cricket ball, a tennis ball and a rubber ball
- A metre ruler or a tape fixed on a wall
- A camera to film the bounce in slow motion
- Patches of concrete, grass and dry sand
- A notebook
- A friend to read the height
Safety
For this project: Throw balls gently and keep faces clear of a hard cricket ball. An adult should supervise play on the ground.
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 a tape measure on a wall or stick near each surface so its zero is at the ground level.
Hold one ball with its bottom at the 1 m mark and drop it, without throwing.
Film in slow motion and read the highest point of the first bounce.
Repeat three times for each ball and surface, writing each height.
Calculate the average rebound height and the ratio rebound divided by 1 m.
Draw a grouped bar graph with surfaces on the axis and a bar colour for each ball.
Rank the ball and surface combinations from best to worst bounce.
Write an explanation for each ranking using the idea of energy loss.
What you should see
A table of bounce ratios showing that, for example, the rubber ball on concrete bounces highest and any ball on sand barely bounces. The cricket ball gives a different result from the tennis ball.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Where does the bounce energy go?
Into sound, heat and shaping the ground or the ball.
2What do we mean by a ratio of heights?
Rebound height divided by drop height.
3Why drop and not throw?
Throwing changes the starting speed, which is a different variable.
4Why do bowlers care about the pitch?
A hard pitch gives a higher, faster bounce.
Take it further
Drop from three heights and check that the ratio stays about the same for the same ball and surface.
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.
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