Energy, Work and Power: Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Energy, Work and Power revision notes

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Section A

1. Name six energy stores and give an example of each. [6]

2. State the principle of conservation of energy. [2]

Section B

3. A crane lifts a 450 kg load through 18 m in 25 s. Take g = 9.8 N kg⁻¹.

(a) Calculate the work done against gravity. [3] (b) Calculate the useful power output. [2] (c) The motor draws 4.5 kW. Calculate the efficiency. [3] (d) Suggest two reasons why the efficiency is less than 100%. [2]

4. A 0.25 kg ball is dropped from 2.0 m and rebounds to 1.4 m.

(a) Calculate its gravitational potential energy at 2.0 m. [2] (b) Calculate its speed just before impact, assuming no air resistance. [3] (c) Calculate the energy dissipated in the bounce and state where it goes. [3]

5. Explain the difference between energy transferred usefully and energy dissipated, and explain why “energy is wasted” is a misleading phrase. [4]

6. (Cross-topic — the underlying mechanisms (conduction, convection, infrared radiation) belong to thermal physics, not this energy, work and power subtopic.) Explain three methods of reducing unwanted energy transfer from a house, giving the mechanism in each case. [6]

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Section C

7. A car of mass 1200 kg accelerates from 10 m s⁻¹ to 20 m s⁻¹.

(a) Calculate its kinetic energy at each speed. [4]

(b) The speed only doubled, yet the kinetic energy increased by much more than double. Explain why. [2]

8. A worker pushes a box with a force of 55 N across a warehouse floor, moving it 12 m in the direction of the force.

(a) Calculate the work done. [2]

(b) State the energy transfer taking place, and explain why work done and energy transferred are measured in the same unit. [2]

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Answers

1. Any six, 1 mark each: kinetic — a moving car; gravitational potential — a raised mass; elastic potential — a stretched spring; chemical — food or fuel; thermal (internal) — a hot object; nuclear — an unstable nucleus; electrostatic — a charged capacitor.

2. Energy cannot be created or destroyed, only transferred from one store to another [1]; the total energy in a closed system remains constant [1].

3. (a) W = mgh = 450 × 9.8 × 18 [1] [1] = 79 400 J [1]. (b) P = W ÷ t = 79 380 ÷ 25 [1] = 3180 W [1]. (c) Efficiency = useful output ÷ total input = 3180 ÷ 4500 [1] [1] = 0.707 = 70.7% [1]. (d) Friction in the cables, pulleys and bearings transfers energy to the surroundings as heat [1]; sound is produced, and some energy is used lifting the hook and cable itself rather than the load [1].

4. (a) E = mgh = 0.25 × 9.8 × 2.0 [1] = 4.9 J [1]. (b) All the GPE becomes KE, so ½mv² = 4.9 [1]; v² = 2 × 4.9 ÷ 0.25 = 39.2 [1]; v = 6.3 m s⁻¹ [1]. (c) GPE at 1.4 m = 0.25 × 9.8 × 1.4 = 3.43 J [1]; energy dissipated = 4.9 − 3.43 = 1.47 J [1]; it is transferred to the internal energy of the ball and the floor, and to sound, as the ball deforms on impact [1].

5. Useful energy transfer is the transfer the device is designed to produce — for example the kinetic energy output of a motor [1]. Dissipated energy is transferred to the surroundings, usually as heat, in a form that is spread out and cannot easily be recovered [1]. “Wasted” is misleading because the energy still exists — none has been destroyed [1]; it has simply become spread thinly among the particles of the surroundings, so it is no longer useful [1].

6. Any three, 2 marks each: loft insulation — traps air, which is a poor conductor, so it reduces conduction and convection through the roof [1] [1]. Cavity wall insulation — foam in the cavity prevents convection currents forming in the air gap, which would otherwise carry heat across [1] [1]. Double glazing — the gap between the panes, often evacuated or filled with argon, greatly reduces conduction and convection through the window [1] [1]. Draught excluders — prevent warm air escaping and cold air entering by bulk movement of air [1] [1]. Reflective foil behind radiators — reflects infrared radiation back into the room rather than letting it heat the external wall [1] [1].

7. (a) At 10 m s⁻¹: Ek = ½mv² = ½ × 1200 × 10² [1] = 60 000 J [1]. At 20 m s⁻¹: Ek = ½ × 1200 × 20² [1] = 240 000 J [1].

(b) Kinetic energy depends on v², not v, so doubling the speed quadruples the kinetic energy [1] — a much larger increase than the speed change alone would suggest [1].

8. (a) W = Fd = 55 × 12 [1] = 660 J [1].

(b) Chemical energy in the worker’s muscles is transferred, largely to thermal energy (heat) in the box and floor as it is pushed against friction [1]. Doing work on an object is precisely how energy is transferred into or out of it by a force, so both quantities are measured in the same unit, the joule [1].

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Where marks are usually lost

• Omitting the field strength g and using mass alone as if it were the weight — the force worked against when lifting is the weight, mg, so GPE = mgh (mass × field strength × height).
• Expressing efficiency greater than 1 or above 100%.
• Saying energy is “lost” rather than dissipated.
• Explaining insulation without naming the transfer mechanism it blocks.
• Forgetting that kinetic energy depends on the square of speed, not speed itself.
• Confusing work done with force alone — omitting the distance moved in the direction of the force.