Quick answer: An energy transfer is the movement of energy from one store to another or its change from one form to another. This grade 7 science lab on energy transfers gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How much of a ball’s stored energy survives a bounce? This lab measures drop and bounce heights to calculate the gravitational potential energy before and after, the wasted energy and the efficiency of the bounce.
Level: Grade 7 science (Year 8 · Class 7) · Curriculum: MS-PS3-5
Aim
To find what fraction of its gravitational potential energy a tennis ball keeps after one bounce, and to calculate the energy wasted.
Equipment
- A tennis ball of mass 57 g (0.057 kg)
- A metre rule fixed upright against a wall, or a long tape measure
- A phone to film each bounce in slow motion
- A hard, flat floor
Safety
Keep the floor clear of bags and cables so nobody trips. Retrieve the ball straight away after each drop and keep your head away from the ball when it comes back up.
Method
- Hold the bottom of the ball level with the 40 cm mark and release it without pushing.
- Film the drop with the phone level with the rule. Play the film back frame by frame and read the highest point reached after the first bounce (bottom of the ball).
- Repeat three times and calculate the mean bounce height.
- Repeat for drop heights of 60, 80, 100 and 120 cm.
Results
Sample measurements from one group are shown below.
| Drop height (m) | Bounce 1 (m) | Bounce 2 (m) | Bounce 3 (m) | Mean bounce (m) |
|---|---|---|---|---|
| 0.40 | 0.24 | 0.25 | 0.23 | 0.24 |
| 0.60 | 0.35 | 0.37 | 0.36 | 0.36 |
| 0.80 | 0.48 | 0.50 | 0.49 | 0.49 |
| 1.00 | 0.60 | 0.62 | 0.61 | 0.61 |
| 1.20 | 0.71 | 0.73 | 0.72 | 0.72 |
Analysis questions
Question 1 Variables
1. State the independent variable.2. State the dependent variable.
3. Explain why the ball is dropped, not thrown.
Question 2 Energy before and after
Use g = 9.8 N/kg. For the 1.00 m drop, calculate the gravitational potential energy (a) at the release point and (b) at the top of the bounce.Question 3 Wasted energy
Calculate how much energy was wasted in the 1.00 m drop and name where it went.Question 4 Efficiency of the bounce
Calculate the efficiency of the bounce for each drop height, taking the bounce energy as useful. Give each answer to the nearest whole number.Question 5 Graph
Plot bounce height (y-axis) against drop height (x-axis). Describe the graph and calculate its gradient using the point (1.20, 0.72).Question 6 Prediction
Predict the mean bounce height for a drop from 1.50 m and calculate the energy wasted in that bounce.Question 7 Reliability
Look at the three bounces from 0.80 m. State the range and suggest two ways to improve the accuracy.Question 8 Different balls
Predict how the efficiency would change for a softer ball that does not bounce well. Explain using energy transfers.
