This set mixes the five skills from work, energy and efficiency: force versus energy, work done, energy stores, efficiency and explaining energy loss. All questions and data are original and invented for practice. They are not from any past paper.
Take g = 10 N/kg throughout. Work each question on paper first, write units in every answer, then open the worked answer. Keep a note of the ones you miss, and the mistake log and retest queue can help you sort them by error type.
Questions, easy to harder
Q1. Which of these are forces and which are energies: 50 N, 50 J, 12 N, 3 kJ?
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Newtons measure force, so 50 N and 12 N are forces. Joules and kilojoules measure energy, so 50 J and 3 kJ are energies. Note that 3 kJ = 3000 J.
Q2. A pupil pushes a wall with a force of 300 N for ten seconds. The wall does not move. How much work is done on the wall?
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The wall does not move, so the distance moved in the direction of the force is 0 m. Work = 300 × 0 = 0 J. The pupil’s muscles still transfer chemical energy to thermal energy, but the wall gains none.
Q3. A force of 80 N moves a sledge 6.5 m in the direction of the force. Calculate the work done.
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W = Fd = 80 × 6.5 = 520 J. Check: 80 × 6 = 480, plus 80 × 0.5 = 40, total 520.
Q4. A 4.0 kg box is lifted at a steady speed from the floor to a shelf 1.2 m high. How much work is done on the box?
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Lifting force = weight = 4.0 × 10 = 40 N. Work = 40 × 1.2 = 48 J. This is also the gain in gravitational potential energy, mgh = 4.0 × 10 × 1.2 = 48 J.
Q5. A force of 25 N pushes a cart 80 cm. Calculate the work done.
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Convert: 80 cm = 0.80 m. W = 25 × 0.80 = 20 J. Using 80 directly would give 2000, which is wrong by a factor of 100.
Q6. A 0.80 kg bucket is raised 2.5 m. It is then released and falls freely. Ignoring air resistance, find (a) the gravitational potential energy gained and (b) the speed just before it hits the ground.
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(a) mgh = 0.80 × 10 × 2.5 = 20 J.
(b) All 20 J becomes kinetic energy: 20 = ½ × 0.80 × v² = 0.40 × v². So v² = 50 and v = √50 = 7.1 m/s (2 significant figures).
Q7. A roller-coaster car of mass 500 kg starts from rest on a hill 20 m above the lowest point of the track. Ignoring friction, find its speed at the lowest point.
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GPE lost = 500 × 10 × 20 = 100 000 J. This equals the KE at the lowest point: 100 000 = ½ × 500 × v² = 250 × v². So v² = 400 and v = 20 m/s. Notice that the mass cancels, since v² = 2gh = 2 × 10 × 20 = 400.
Q8. An electric motor takes in 240 J and gives 180 J of useful kinetic energy. Find the efficiency and the wasted energy.
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Efficiency = 180 ÷ 240 = 0.75 = 75%. Wasted energy = 240 − 180 = 60 J.
Q9. A crane lifts a 250 kg load 12 m using 48 kJ of electrical energy. Calculate its efficiency.
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Useful energy = mgh = 250 × 10 × 12 = 30 000 J = 30 kJ. Input = 48 kJ. Efficiency = 30 ÷ 48 = 0.625 = 62.5%. Both quantities are in kJ here, so the units match.
Q10. A machine has an efficiency of 35% and takes in energy at a rate of 2.0 kW. Find (a) the useful output power and (b) the useful energy output in 60 s.
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(a) Input = 2.0 kW = 2000 W. Useful power = 0.35 × 2000 = 700 W.
(b) Energy = power × time = 700 × 60 = 42 000 J (42 kJ).
Q11. A 0.25 kg ball is dropped from 2.0 m and rebounds to 1.5 m. Calculate the energy transferred to thermal and sound stores in the bounce, and the efficiency of the bounce.
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Start GPE = 0.25 × 10 × 2.0 = 5.0 J. Rebound GPE = 0.25 × 10 × 1.5 = 3.75 J. Energy transferred to thermal and sound = 5.0 − 3.75 = 1.25 J. Efficiency = 3.75 ÷ 5.0 = 0.75 = 75%.
Q12. A student writes: “The brakes use up the car’s kinetic energy.” Rewrite the sentence so that it is correct physics.
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“Friction in the brakes transfers the car’s kinetic energy to the thermal store of the brake pads, discs and surroundings, and some to sound. The total energy is unchanged, but the thermal energy is spread out and not useful.”
If you got these wrong
| What went wrong | Questions | Go back to |
|---|---|---|
| Mixed up N and J, or said work is done with no movement | Q1, Q2 | Distinguish energy transfer from force |
| Wrong distance, unit not converted, mass used as force | Q3, Q4, Q5 | Calculate work along a displacement |
| Forgot to square the speed or used the wrong store | Q6, Q7 | Track energy stores in a simple system |
| Mixed units or inverted the efficiency fraction | Q8, Q9, Q10 | Calculate efficiency with consistent quantities |
| Wrote that energy is lost or used up | Q11, Q12 | Explain an energy loss without saying energy disappears |
What should I do next?
Redo any question you missed after a day, with a fresh set of numbers of your own. Once the set feels steady, move on to power and energy resources.
A teacher in online one-to-one Physics tuition can go through your working and suggest what to practise next.