A student investigates how well different balls bounce.
She drops five different balls from the same height and measures the height the balls bounce.
She repeats the experiment three times for each ball.

Her results are shown in Table 20.1.
Ball | Drop height (cm) | Bounce height (cm) | Mean bounce height (cm) | ||
1st reading | 2nd reading | 3rd reading | |||
Red | 100 | 75 | 77 | 73 | 75 |
Blue | 100 | 61 | 62 | 60 | 61 |
Green | 100 | 60 | 31 | 58 |
|
White | 100 | 84 | 86 | 85 | 85 |
Yellow | 100 | 26 | 24 |
| 26 |
Table 20.1
Calculate the mean bounce height for the green ball.
Answer = .......................... cm
The student forgot to write down one of the bounce heights for the yellow ball.
Suggest the missing result for the yellow ball.
Answer = .........................cm
Evaluate the reliability of the results.
Suggest how she could have improved her experiment.
Higher Tier Only
The student suggests that 15% of the white ball’s initial energy was not transferred usefully.
i) Show that her suggestion is correct and suggest where the energy has been transferred to.
Use calculations and the information in Table 19.1 to help you answer.
ii) How could the efficiency of the ball be improved?
Explain how energy is transferred and lost from the ball when it bounces.
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