Year 9 Science · Unit 3 · Lesson 6
Challenge Worksheet
Learning Goals
1. The equation E = P × t links energy, power and time. Write it in words, and give the unit of each quantity.
Marking practice: Kai’s homework
(a) A forklift lifts a 300 kg pallet up 2 m. W = 300 × 2 = 600 J
(b) A 1,500 W sandwich press runs for 2 minutes. E = 1,500 × 2 = 3,000 J
(c) A tractor does 90,000 J of work in 30 s. P = 30 ÷ 90,000 = 0.00033 W
(d) A 40 N push slides a desk 250 cm. W = 40 × 250 = 10,000 J
2. Each of Kai’s answers has one error. Say what Kai did wrong in each, and write the correct answer with its unit. Use g = 10 N/kg.
| Part | What Kai did wrong | Correct answer |
|---|---|---|
| (a) | ||
| (b) | ||
| (c) | ||
| (d) |
3. A rescue helicopter’s winch lifts a 75 kg surfer 20 m up to the helicopter in 25 s. Use g = 10 N/kg. (a) Calculate the work done on the surfer and the winch’s useful power. (b) The winch motor draws 1,500 W of electrical power during the lift. Calculate the electrical energy it uses, and the percentage of that energy that ends up as the surfer’s gravitational potential energy.
(a) Work done J
(a) Useful power W
(b) Electrical energy used J
(b) Percentage %
Data: an exercise-bike sprint
Four riders each pedalled an exercise bike as hard as they could for 2 minutes (120 s). The bike’s display showed the work each rider did.
| Rider | Work done (kJ) | Average power (W) |
|---|---|---|
| A | 36 | 300 |
| B | 27 | |
| C | 48 | |
| D | 30 |
Source: sample class results.
4. Complete Table 1. Rider A is done for you: 36,000 J ÷ 120 s = 300 W.
5. A clothes dryer is rated at 2,400 W. (a) How much energy does it use in a 10-minute run? (b) How long would rider C have to keep pedalling at the same average power to supply that much energy? Give your answer in minutes.
(a) Energy J
(b) Time minutes
6. Use your answer to Question 5 to explain why household appliances run on mains electricity rather than on human pedal power.
7. A student claims: “Power and energy are the same thing. A 2,000 W oven always uses more energy than a 250 W slow cooker.” A family cooks a roast in the oven for 45 minutes. Another night they cook a stew in the slow cooker for 8 hours. Evaluate the student’s claim.
Energy used by the oven J
Energy used by the slow cooker J