Work, power and efficiency · Grade 9
Work is done when a force moves something: energy is transferred. Power is how quickly that energy is transferred, measured in watts. A powerful crane lifts the same load as a weak one, just faster. Efficiency tells you how much of the power is useful.
Key facts
- Work done W = Fs (force × distance moved in the direction of the force). 1 J = 1 N m.
- Work done against gravity = mgh.
- Power P = E/t = W/t, in watts (1 W = 1 J/s).
- At steady speed, P = Fv (driving force × velocity).
- Efficiency = useful power out ÷ power in.
- For a force at angle θ to the motion: W = Fs cos θ. Work = area under a force–distance graph.