Year 9 Science · Unit 3 · Lesson 6
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Learning Goals
1. What does power measure, and what is its unit? How is power different from work?
Scenario: two cranes on a Sydney building site
Two cranes each lift an identical 500 kg steel beam to the same height of 10 m. Crane A takes 20 seconds. Crane B takes 3 minutes 20 seconds (200 s). Take g as 10 N/kg.
2. Calculate (a) the weight of the beam, (b) the work done by each crane, and (c) the power of each crane.
(a) Weight N
(b) Work done by each crane J
(c) Power of crane A W
(c) Power of crane B W
3. An electric motor does 24,000 J of work in 2 minutes. Calculate its power output. Show your working and the unit.
Power W
4. A hiker takes a long zig-zag trail up a hill. Her friend climbs the short, steep path to the same summit. Explain why they do the same work against gravity, and why the zig-zag trail needs a smaller force at each step. Use W = F × d.
Investigation: a stair-climb power test
Four students ran up the same flight of stairs. The vertical height of the stairs is 3.0 m. A partner timed each run. Take g as 10 N/kg.
| Student | Mass (kg) | Time (s) | Work done (J) | Power (W) |
|---|---|---|---|---|
| Ana | 50 | 4.0 | 1,500 | 375 |
| Ben | 60 | 5.0 | ||
| Chloe | 45 | 3.0 | ||
| Dev | 70 | 6.0 |
Source: sample class results.
5. Complete Table 1. Ana’s row is done for you: work = 50 × 10 × 3.0 = 1,500 J, and power = 1,500 ÷ 4.0 = 375 W.
6. Rank the four students from most powerful to least powerful.
7. Dev did the most work, but he had the lowest power. Explain how both of these can be true.
8. A school replaces the motor in its goods lift. On each trip the lift raises 800 kg (cabin and load) through 12 m. The old motor took 40 s per trip. The new motor takes 16 s. The principal says: “The new motor is faster, so it must do more work on every trip.” Evaluate the principal’s statement. Use calculations of work and power.
Work done per trip J
Power of old motor W
Power of new motor W