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Work, energy and power

Calculate work done, changes in gravitational and kinetic energy, and power in everyday systems.

12 free questions · Notes · Worked solutions

Practise this topic

What you will practise

  • Calculate work done by a force through a distance.
  • Use gravitational potential and kinetic energy equations.
  • Calculate power as energy transferred per time.
  • Compare useful energy output with input using efficiency.

Revision notes

Work and energy

Work done by a constant force parallel to motion is W=Fd. Work done transfers energy, so work and energy are measured in joules. If force is not parallel, use the component along the displacement.

Energy stores

Gravitational potential energy change is ΔEₚ=mgΔh. Kinetic energy is Eₖ=½mv². Use mass in kilograms and speed in metres per second.

Power and efficiency

Power is the rate of energy transfer: P=E/t, measured in watts. Efficiency=useful output energy/total input energy; multiply by 100 for a percentage.

Common checks

Doubling speed quadruples kinetic energy. A higher object has more gravitational potential energy relative to the chosen reference level. Do not confuse energy (J) with power (W).

A worked example

A 2 kg book is lifted vertically by 3 m. Using g=10 N/kg, find the gain in gravitational potential energy. If the lift takes 2 s, find the average power.

  1. The height change is 3 m and the mass is 2 kg.
  2. Gain in GPE=mgh=2×10×3=60 J.
  3. Power=energy/time=60÷2=30 W.

60 J; 30 W

Try it yourself

These are original practice questions, not official past-paper questions. Practice points count correct final answers; working is not automatically assessed.

Choose a question to begin.
Question 1 · Standard1 practice point

A 10 N force moves an object 3 m in the force's direction. How much work is done?

Need a hint?

Work=force×distance.

Worked solution
  1. W=10×3=30 J.
Question 2 · Standard1 practice point

What is the kinetic energy of a 2 kg object moving at 3 m/s?

Need a hint?

Use Eₖ=½mv².

Worked solution
  1. Eₖ=½×2×3²=9 J.
Question 3 · Standard1 practice point

A machine transfers 200 J in 4 s. What is its power?

Need a hint?

Power=energy/time.

Worked solution
  1. P=200÷4=50 W.
Question 4 · Standard1 practice point

A device receives 100 J and provides 80 J of useful output. What is its efficiency?

Need a hint?

Useful output divided by input, then multiply by 100%.

Worked solution
  1. Efficiency=80/100=0.8.
  2. As a percentage, this is 80%.
Question 5 · Foundation1 practice point

A 15 N force pushes a box 4 m in the direction of the force. Find the work done.

Need a hint?

Multiply force by distance.

Worked solution
  1. W=Fd=15×4.
  2. W=60 J.
Question 6 · Standard2 practice points

Find the gravitational potential energy gained by a 3 kg object lifted 2 m, using g=10 N/kg.

Need a hint?

Use ΔEp=mgΔh.

Worked solution
  1. ΔEp=3×10×2.
  2. ΔEp=60 J.
Question 7 · Standard2 practice points

A 4 kg object moves at 5 m/s. Find its kinetic energy.

Need a hint?

Square 5 before multiplying by half the mass.

Worked solution
  1. Ek=½×4×5².
  2. Ek=2×25=50 J.
Question 8 · Standard2 practice points

A motor transfers 900 J in 30 s. Find its average power.

Need a hint?

Divide energy transferred by time.

Worked solution
  1. P=E/t=900/30.
  2. P=30 W.
Question 9 · Foundation2 practice points

A 50 kg person climbs a vertical height of 4 m. Find the gain in gravitational potential energy using g=10 N/kg.

Need a hint?

Use mass×g×height.

Worked solution
  1. ΔEp=50×10×4.
  2. ΔEp=2000 J.
Question 10 · Standard2 practice points

A lamp uses 500 J of electrical energy and transfers 75 J as light. Find its efficiency as a percentage.

Need a hint?

Divide useful output by input and multiply by 100.

Worked solution
  1. Efficiency=75/500=0.15.
  2. 0.15×100=15%.
Question 11 · Standard2 practice points

A 1000 W kettle operates for 120 s. Find the energy it transfers in joules.

Need a hint?

Use E=Pt.

Worked solution
  1. E=1000×120.
  2. E=120 000 J.
Question 12 · Foundation2 practice points

A 0.8 kg ball is moving at 10 m/s. Find its kinetic energy.

Need a hint?

Use half the mass times speed squared.

Worked solution
  1. Ek=½×0.8×10².
  2. Ek=0.4×100=40 J.

Coverage and sources

Selected IGCSE concepts, with a mix of introductory and more demanding questions. Check your own specification. AI drafted; independent teacher review is pending. Editorial details.

Cambridge IGCSE Physics 0625 syllabus

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