Year 7 Science · Unit 3 · Lesson 13
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Learning Goals
1. Name the two kinds of potential energy in this lesson. For each one, say what a force must do to an object to store it.
Odd one out
Each set has a rule. Circle the one example that does not fit its rule, and give a reason.
| Circle the odd one out, then write your reason in the box below it |
|---|
| 1. Rule: the force does work on the object. kicking a still ball · lifting a bag onto a bench · pushing on a wall that does not budge · pulling back a slingshot band |
| 2. Rule: energy is stored as elastic potential energy. a squashed pen spring · a stretched bungee cord · a pot plant on a balcony · a drawn-back bow |
| 3. Rule: friction turns movement energy into heat. hot bike brake pads · hands rubbed together · a wound-up toy car speeding off · legs warming on a slide |
Cause and effect: a slingshot
Fill each empty box. Each box is caused by the one before it, and each row carries on from the end of the row above.
Order the energy changes: a half-pipe run
A skateboard rider at a skate park in Western Sydney pumps at the bottom of a half-pipe, then rides up and down it many times without pumping again. Number the steps from 1 (first) to 6 (last).
| Step (1–6) | What happens |
|---|---|
| The rider pauses at the top of the ramp, where the energy is mostly gravitational potential energy. | |
| After many runs, each climb a little lower than the last, the rider rolls to a stop near the middle. | |
| The rider speeds across the flat bottom with a lot of kinetic energy. | |
| Gravity pulls the rider back down the ramp, and the rider speeds up again. | |
| The rider pumps with their legs, using chemical energy in their muscles. | |
| The rider climbs the ramp and slows down as kinetic energy is stored as gravitational potential energy. |
2. Two students push on a parked car with the handbrake off. Student A pushes hard, but the car does not move. Student B pushes, and the car rolls forward. Which student does work on the car? Explain using energy transfer.
Read the data: a rubber-band car
Scenario: STEM day
A class stretched the rubber band of a toy car by different amounts, let it go on a flat floor, and measured how far it travelled. The car, band and floor stayed the same.
| Stretch of the band (cm) | Distance travelled (m) |
|---|---|
| 2 | 0.6 |
| 4 | 1.3 |
| 6 | 1.8 |
| 8 | 2.5 |
| 10 | 3.1 |
Source: sample class results, average of three runs.
3. Use Table 1 to predict how far the car travels with a 5 cm stretch. Explain how you got your answer.
Answer m
4. Explain the trend in Table 1 using energy. Say what is stored, what it becomes, and why the car always stops in the end.
5. A bungee jumper steps off a high bridge. She falls, the cord stretches and slows her, and then it pulls her back up. She bounces up and down a few times, each bounce lower than the last, until she hangs still. Trace the energy changes, naming the forces that do the work.
Question 1
Elastic: stretch or squash it (1). Gravitational: lift it against gravity (1). [2 marks]
Odd one out
1 mark per row, with reason. 1: the wall (no movement, no work). 2: the pot plant (gravitational, not elastic). 3: the toy car (elastic → kinetic, no friction heating). [3 marks]
Cause and effect: a slingshot
Box 1: the band stores elastic potential energy (1). Box 2: released, it pushes the stone: stored → kinetic (1). Box 3: friction turns kinetic energy into heat and sound (1). [3 marks]
Order the energy changes: a half-pipe run
Reading down the table as printed, the correct step numbers are 4, 6, 2, 5, 1, 3. In order: pumps; speeds across; climbs; pauses; rolls down; stops.
Question 2
Student B (1): the car moves, so energy is transferred to it as kinetic energy (1). A’s push moves nothing: no work (1). [3 marks]
Question 3
About 1.55 m; accept 1.5–1.6 m (1). 5 cm is halfway between 4 cm (1.3 m) and 6 cm (1.8 m) (1). [2 marks]
Question 4
More stretch stores more elastic energy (1), giving more kinetic energy, so it goes further (1). Friction turns it into heat and sound, so it stops (1). [3 marks]
Question 5
Guide, 1 mark each: gravitational potential energy at the start; gravity does work → kinetic; the cord stores elastic energy; it pulls her up (elastic → kinetic → gravitational); heat and sound make each bounce lower; energy conserved. [6 marks]
Model answer: Her gravitational potential energy becomes kinetic energy as gravity does work. The cord stores it as elastic potential energy, then pulls her up, back to kinetic and gravitational energy. Air resistance and the cord turn some into heat and sound each bounce, so she rises less. None is destroyed.
Common error: “A did work because it was hard.”