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Work, power and efficiency Cambridge IGCSE Combined Science revision

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In plain words

Work is energy transferred by a force: push something along and the work you've done is force × distance. Power is how fast you do it: joules per second, called watts. Two people can do the same work climbing the stairs; the one who gets there first is the more powerful.

Three things to know

  1. Work done = force × distance moved in the direction of the force (W = Fd). Work done equals energy transferred.
  2. Power = work done ÷ time taken (P = W/t). One watt is one joule per second.
  3. Efficiency = useful energy (or power) output ÷ total input, × 100%.

Worked example

A motor lifts a 200 N load through 3 m in 5 s. Find its power.

  1. Work done = force × distance = 200 × 3 = 600 J.
  2. Power = work ÷ time = 600 ÷ 5.
  3. = 120 W.

Tips and tricks

  • Efficiency can never be more than 100%. If yours is, the numbers are upside down: it's useful output ÷ total input.
  • Time must be in seconds. Change minutes first.
6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Work, power and efficiency: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

  1. A force of 20 N moves a box 5 m. How much work is done?
    • 4 J
    • 25 J
    • 100 J (the answer)
    • 500 J

    W = Fd = 20 × 5.

  2. What is power?
    • the force used to do work
    • the total energy transferred
    • the rate at which energy is transferred (the answer)
    • the distance moved each second

    Power is energy transferred per second.

  3. A machine does 3000 J of work in 60 s. What is its power?
    • 20 W
    • 50 W (the answer)
    • 3060 W
    • 180 000 W

    P = W ÷ t = 3000 ÷ 60.

  4. A motor is supplied with 500 J and does 400 J of useful work. What is its efficiency?
    • 20%
    • 25%
    • 80% (the answer)
    • 125%

    400 ÷ 500 × 100 = 80%.

  5. What usually happens to the energy a machine wastes?
    • It disappears.
    • It heats the surroundings. (the answer)
    • It is stored in the machine.
    • It turns back into fuel.

    Wasted energy spreads out as thermal energy.

  6. A weightlifter holds a 1000 N bar still above her head. How much work is she doing on the bar?
    • 0 J (the answer)
    • 500 J
    • 1000 J
    • 2000 J

    The bar is not moving, so no work is done on it.

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