Quick answer: Efficiency is the fraction of the total input energy that becomes useful output: efficiency = useful output ÷ total input, often shown as a percentage. This Physics 9-10 lab on energy efficiency gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How well does a ball return its energy after it hits the ground? In this lab a tennis ball is dropped onto four surfaces and the rebound height is measured. The ratio of heights gives the efficiency of the bounce, and the rest of the energy is shown to be wasted. It uses simple equipment at school or at home.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE P1, HS-PS3-4
Aim
To compare how efficiently a ball returns energy when it bounces on different surfaces.
Equipment
- Tennis ball of mass 0.058 kg
- Metre rule fixed upright, a phone for slow-motion video
- Four test surfaces: concrete slab, wooden board, carpet square, tray of sand
- Balance
Safety
Drop the ball only over the test surface and keep the area clear. Do not stand on chairs: fix the ruler to a wall or stand. Wipe up sand and keep it out of the eyes.
Method
- Fix the metre rule upright beside the first test surface. Place the phone level with the rule to film the drop.
- Release the ball from rest with its bottom at the 1.00 m mark. Do not throw it down.
- Watch the slow-motion video and read the highest point reached after the first bounce. Repeat three times and take the mean rebound height h.
- Repeat for the other three surfaces, using the same ball and the same release height.
- Calculate the energy at release, the energy at rebound, and the efficiency = rebound height ÷ release height × 100 %.
Results
Sample results for a drop height of 1.00 m and g = 9.8 N/kg. The release energy is m g H = 0.058 × 9.8 × 1.00 = 0.568 J.
| Surface | Mean rebound height (m) | Efficiency (%) | Energy returned (J) |
|---|---|---|---|
| Concrete | 0.58 | 58 | 0.330 |
| Wooden floor | 0.52 | 52 | 0.296 |
| Carpet | 0.38 | 38 | 0.216 |
| Sand tray | 0.12 | 12 | 0.068 |
Analysis questions
Question 1 Energy at release
Calculate the gravitational potential energy of the ball at the 1.00 m mark.Question 2 Concrete result
Use the concrete row to calculate the energy returned and the energy wasted in the first bounce.Question 3 Where does the energy go?
Name two forms in which the wasted energy is transferred when the ball hits the surface.Question 4 Rank the surfaces
Rank the four surfaces by efficiency and explain the order.Question 5 Predict a second bounce
Assume that each bounce on concrete has the same efficiency. Predict the rebound height after the second bounce.Question 6 Reading uncertainty
The rebound height is read to ±0.02 m. Calculate the percentage uncertainty for the concrete and the sand results, and state which is less reliable.Question 7 Improve the lab
Suggest two improvements to make the investigation more reliable.
