IGCSE Physics Work, Power & Efficiency Notes
Work is energy transferred by a force; power is how fast it is transferred; efficiency measures how much is useful. Learn all three equations.
Key concepts
- Work done
- Energy transferred when a force moves an object: W = F × d, in joules.
- Power
- The rate of doing work or transferring energy: P = W ÷ t, in watts.
- Efficiency
- Useful output energy divided by total input energy, expressed as a fraction or percentage.
Key formulas
Work done
W = F × d
force (N) × distance moved (m).
Power
P = W ÷ t
work (J) ÷ time (s).
Efficiency
efficiency = useful out ÷ total in × 100%
Worked example
A crane lifts a 200 N load 5 m in 10 s. Find the power.
- 1Work done: W = F × d = 200 × 5 = 1000 J.
- 2Power equation: P = W ÷ t.
- 3Substitute: P = 1000 ÷ 10.
- 4Calculate: P = 100 W.
Answer: P = 100 W
Common mistakes
✗ Using the wrong distance in W = F × d.
✓ Use the distance moved in the direction of the force.
✗ Confusing work (J) with power (W).
✓ Work is total energy transferred; power is the rate per second.
✗ Forgetting to multiply efficiency by 100 for a percentage.
✓ Multiply the fraction by 100 when a percentage is asked for.
Quick check
Try these, then reveal the answers.
State the equation for work done.Reveal answer
W = F × d.
A machine does 600 J of work in 3 s. What is its power?Reveal answer
200 W
What are the units of work and power?Reveal answer
Joules (J) and watts (W).
Exam tips
- Calculate work first, then use it to find power.
- Check the distance is measured along the direction of the force.
- Show clear steps to secure method marks even if arithmetic slips.
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Frequently asked questions
What is the difference between work and power?+
Work is the total energy transferred by a force; power is how fast that energy is transferred.
How do I calculate efficiency as a percentage?+
Divide useful energy output by total energy input, then multiply by 100.
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