Work, power and efficiency

Work is done when a force moves something: energy is transferred. Power is how fast the energy is transferred. Efficiency says how much of the energy ends up being useful.

Key facts

  • Work done = force × distance moved (J).
  • Power = energy transferred ÷ time = work done ÷ time (W).
  • 1 W = 1 J per second.
  • Efficiency = useful energy out ÷ total energy in.

Worked examples

Example 1

A crane lifts a 500 N load 12 m in 20 s. Find the work done and the power.

  1. W = 500 × 12 = 6000 J
  2. P = 6000 ÷ 20 = 300 W

Answer: 6000 J and 300 W

Example 2

A motor uses 800 J to lift a load, giving it 600 J of GPE. Find its efficiency.

  1. 600 ÷ 800 = 0.75 = 75%

Answer: 75%