Year 7 Science · Unit 3 · Lesson 13
Challenge Worksheet
Learning Goals
Find the errors
A student wrote these revision notes. Three of the five notes contain one error each. For each note, write “correct”, or rewrite it so it is true.
| Student’s note | Correct, or your correction |
|---|---|
| 1. A force does work whenever it pushes on an object, even if the object does not move. | |
| 2. Squashing a spring stores elastic potential energy in it. | |
| 3. Friction destroys movement energy, which is why a sliding box stops. | |
| 4. The higher you lift an object, the more gravitational potential energy it stores. | |
| 5. As a ball falls, gravity creates new kinetic energy in it. |
Compare the energy changes
For each situation, name the force that acts and the energy change it causes.
| Situation | Force that acts | Energy change |
|---|---|---|
| A diver climbs the ladder to a 10 m platform. | ||
| A trampoline mat stretches as a gymnast lands on it. | ||
| A car’s tyres skid to a stop and leave hot rubber marks. |
1. Two identical balls are lifted onto shelves. Ball A is lifted twice as high as Ball B. Both are dropped. Predict which ball is moving faster just before it hits the floor, and explain using stored energy and movement energy.
Read the data: the bouncing ball
Scenario: Callum’s drop test
Callum drops a rubber ball from different heights onto a concrete path and films it to measure how high it rises on the first bounce. The ball never bounces back to the height it was dropped from.
| Drop height (cm) | 50 | 100 | 150 | 200 |
|---|---|---|---|---|
| First bounce height (cm) | 35 | 70 | 104 | 140 |
Source: made-up results for this worksheet.
2. For the 100 cm and 200 cm drops, calculate the bounce height as a percentage of the drop height. What do you notice?
100 cm drop %
200 cm drop %
3. Use the pattern to predict the first bounce height for a 250 cm drop. Show your working.
Answer cm
4. Callum concludes: “The ball loses energy every time it hits the path, so energy is destroyed in a bounce.” Evaluate Callum’s conclusion. Use Table 1 and the idea that energy is conserved.
5. Ava pushes a heavy box across a carpet. Her push makes the box move, so she does work on it. But the moment she stops pushing, the box stops. Where did the energy she transferred go? Explain.
6. An inventor claims to have built a wheel that, after one push, keeps spinning forever and even speeds up on its own, with nobody pushing it again. The inventor says it makes free energy out of nothing. Evaluate the inventor’s claim.
Find the errors
Note 1: work is done only if the object moves (1). Notes 2 and 4 are correct. Note 3: friction turns movement energy into heat and sound; it does not destroy it (1). Note 5: gravitational potential energy becomes kinetic energy; none is created (1). [3 marks]
Compare the energy changes
1 mark per row, both cells. Diver: her push lifts her against gravity; chemical → gravitational potential. Mat: the gymnast’s push stretches it; kinetic → elastic potential. Tyres: friction; kinetic → heat and sound. [3 marks]
Question 1
Ball A (1). Lifted higher, it stores more gravitational potential energy (1), which becomes more kinetic energy, so it is faster (1). [3 marks]
Question 2
70 ÷ 100 × 100 = 70% (1); 140 ÷ 200 × 100 = 70% (1). It always rises to about 70% of the drop height (1). [3 marks]
Question 3
70% of 250 cm = 175 cm; accept 170–178 cm with working (1). [1 mark]
Question 4
The ball does have less energy after a bounce: it rises only 70% as high, so less gravitational potential energy (1). But energy is conserved, not destroyed (1). The rest becomes heat and sound (the thud) at the path, plus heat from air resistance (1). [3 marks]
Question 5
Friction between box and carpet turned it into heat and sound (1). It still exists, but not as movement energy, so the box stops (1). [2 marks]
Question 6
Marking guide, 1 mark each: judgement (impossible); friction and air resistance act every turn; they turn kinetic energy into heat, so it slows and stops; speeding up needs a force doing work; energy is conserved, not made from nothing; key terms correct. [6 marks]
Model answer: Impossible. Every turn, friction and air resistance turn the wheel’s kinetic energy into heat, so it slows and stops. Speeding up needs a force to do work on it, and nobody pushes. Energy is conserved: no machine makes it from nothing.
Common error: saying energy is “lost” in a bounce.