IGCSE Physics: Energy, Work and Power Practice Questions
Energy is transferred between stores but is never created or destroyed. Work done equals force multiplied by distance moved in the direction of the force, and power is the rate at which work is done or energy is transferred.
Sub-topic 1.7 of Cambridge IGCSE Physics 0625 carries four equations that appear on nearly every paper. The energy conversion question, where gravitational potential energy becomes kinetic energy, is the one that separates grades because it requires equating two expressions. The questions below build up to it.
What you need to know for Energy, Work and Power
- Kinetic energyKE = half x mass x velocity squared. Note that velocity is squared, so doubling the speed multiplies kinetic energy by four.
- Change in gravitational potential energyChange in GPE = mass x gravitational field strength x change in height, written mg delta h.
- Work doneWork done = force x distance moved in the direction of the force, measured in joules. One joule is one newton metre.
- PowerPower = work done divided by time taken, or energy transferred divided by time taken, measured in watts. One watt is one joule per second.
- EfficiencyEfficiency = useful energy output divided by total energy input, expressed as a decimal or multiplied by 100 for a percentage. Efficiency can never exceed 100 per cent.
- Conservation of energyEnergy cannot be created or destroyed, only transferred from one store to another. Energy that is not usefully transferred is usually described as dissipated to the surroundings as thermal energy.
IGCSE Physics Energy, Work and Power questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
A car of mass 1400 kg is travelling at 20 m/s. Calculate its kinetic energy.
Show the worked answer
- Use KE = half x mass x velocity squared.
- Square the velocity first: 20 squared = 400.
- KE = 0.5 x 1400 x 400.
- KE = 280000 J, which is 280 kJ.
A ball of mass 0.20 kg is dropped from a height of 1.8 m. Calculate its speed just before it hits the ground, assuming air resistance is negligible. Take g as 9.8 N/kg.
Show the worked answer
- As the ball falls, gravitational potential energy is transferred to kinetic energy. With no air resistance, all of it is transferred.
- Change in GPE = mg delta h = 0.20 x 9.8 x 1.8 = 3.528 J.
- Set this equal to the kinetic energy: 3.528 = 0.5 x 0.20 x v squared, so v squared = 3.528 divided by 0.10 = 35.28.
- v = square root of 35.28 = 5.94, which is 5.9 m/s to 2 significant figures. Note that the mass cancels, so any mass dropped from this height reaches the same speed.
A crane lifts a 250 kg load through a vertical height of 12 m in 40 s. Calculate the useful power output of the crane. Take g as 9.8 N/kg.
Show the worked answer
- The useful work done is the gain in gravitational potential energy of the load.
- Work done = mg delta h = 250 x 9.8 x 12 = 29400 J.
- Power = work done divided by time taken = 29400 divided by 40.
- Power = 735 W.
An electric motor is supplied with 4800 J of electrical energy and does 1200 J of useful work. Calculate its efficiency as a percentage and state what happens to the remaining energy.
Show the worked answer
- Efficiency = useful energy output divided by total energy input.
- Efficiency = 1200 divided by 4800 = 0.25.
- As a percentage: 0.25 x 100 = 25 per cent.
- The remaining 3600 J is transferred to the surroundings, mostly as thermal energy through friction in the bearings and heating in the wires, and some as sound. The total energy is unchanged, which is why efficiency can never exceed 100 per cent.
Common mistakes in this topic
- Forgetting to square the velocity in the kinetic energy equation.
- Using the distance along a slope instead of the vertical height in a GPE calculation.
- Confusing power with energy. Power is the rate of energy transfer, measured in watts.
- Saying energy is lost. Energy is transferred to the surroundings, usually as thermal energy.
- Giving an efficiency above 100 per cent, which is impossible.
Exam tips
- Learn the four equations and write the relevant one first every time. That line alone is usually a mark.
- In any drop or slide problem, set the change in GPE equal to the kinetic energy gained.
- Check whether the question wants joules, kilojoules, watts or kilowatts before writing the final answer.
- Use the word dissipated rather than lost when describing energy transferred to the surroundings.
- Doubling speed multiplies kinetic energy by four. That fact explains stopping distance questions.
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Energy, Work and Power FAQs
What is the equation for kinetic energy?
Kinetic energy equals half multiplied by mass multiplied by velocity squared. Mass is in kilograms, velocity in metres per second and the energy comes out in joules. Because velocity is squared, doubling the speed of an object multiplies its kinetic energy by four.
How do I calculate the speed of a falling object using energy?
Calculate the change in gravitational potential energy using mass multiplied by g multiplied by the drop in height. If air resistance is negligible, all of it becomes kinetic energy, so set it equal to half multiplied by mass multiplied by velocity squared, rearrange for velocity squared, and take the square root.
What is the difference between work and power?
Work done is the energy transferred when a force moves an object, calculated as force multiplied by distance in the direction of the force, and measured in joules. Power is the rate at which that work is done, calculated as work divided by time, and measured in watts.
How do I calculate efficiency?
Divide the useful energy output by the total energy input, then multiply by 100 for a percentage. Efficiency can never exceed 100 per cent, because the energy that is not usefully transferred is dissipated to the surroundings, usually as thermal energy and sound.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.