Year 9 Science · Unit 3 · Lesson 3
Challenge Worksheet
Learning Goals
1. Name the three quantities that are multiplied together in GPE = mgh, and give the unit of each one.
Find the mistakes: a student’s working
(a) A 45 kg child sits at the top of a slide 3.2 m high. GPE = 45 × 10 × 320 = 144,000 J
(b) A 0.06 kg tennis ball moves at 40 m/s. KE = ½ × 0.06 × 40 = 1.2 J
(c) A motor takes in 400 J and gives out 300 J of kinetic energy. Efficiency = 400 ÷ 300 × 100 = 133%
(d) A toaster is 90% efficient and takes in 2 kJ. Waste = 2 − 1.8 = 0.2 J
2. The student made one mistake in each calculation. Describe each mistake and give the correct answer with its unit. Use g = 10 m/s².
| Part | What is wrong | Correct answer |
|---|---|---|
| (a) | ||
| (b) | ||
| (c) | ||
| (d) |
3. The child in (a) reaches the bottom of the slide at 6 m/s. Calculate the child’s kinetic energy at the bottom, and the energy transformed into thermal energy by friction. What percentage of the child’s GPE became kinetic energy?
Kinetic energy J
Thermal energy J
GPE that became kinetic energy %
Data: two ways to heat a room
A family compares a gas heater with an electric heater. Table 1 shows the energy each heater takes in and the useful heat it gives out.
The electricity for the electric heater comes from a gas-fired power station. The power station turns 50% of the chemical energy in its gas into electrical energy. Ignore energy lost in the power lines.
| Heater | Energy in (J) | Useful heat out (J) | Efficiency (%) |
|---|---|---|---|
| Gas heater | 2,000 (chemical) | 1,700 | 85 |
| Electric heater | 1,200 (electrical) | 1,140 |
Source: sample values for this worksheet.
4. Calculate the efficiency of the electric heater and write it in Table 1.
5. Follow the electric heater’s energy back to the gas. (a) How much chemical energy must the power station burn to supply the heater’s 1,200 J? (b) What is the overall efficiency, from the gas at the power station to useful heat in the room? (c) Which heater gets more useful heat from each joule of gas burned?
(a) J
(b) %
(c)
6. On a sunny morning, the panel in Question 7 delivers 660 J of electricity each second. Using its 22% efficiency, calculate the sunlight energy arriving on it each second, and the energy that is not converted.
Sunlight arriving J
Not converted J
7. Someone claims: “This solar panel is only 22% efficient, so it is a complete waste of money. Nearly 78% of the sunlight hitting it is wasted, so solar panels are basically useless. We would be better off burning natural gas, which gives much more useful energy per dollar.” Evaluate the claim.