AS Level Physics · Work, energy and power. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
Work is done when a force moves an object in the direction of the force. Only the part of the force along the displacement counts, so a force at right angles to the motion does no work.
Energy cannot be created or destroyed. It is only transferred from one form to another, so the total energy before equals the total energy after, as long as you include the energy dissipated as heat by friction or air resistance.
Efficiency is the fraction of the energy input that comes out in a useful form. Power is the work done per unit time. For an object moving at velocity v under a force F, the power is Fv.
In efficiency questions students divide the wrong way round or use the wasted energy. Efficiency is always useful output ÷ total input, so it can never be more than 1 (100%).
Answer: Efficiency = 0.59, which is 59%
Try each one, then open the answer.
Answer: B. W = Fs cos θ = 40 × 12 × 0.866 = 416 J. Only the component of force along the displacement does work.
Answer: C. P = Fv, so F = P/v = 60 000 ÷ 25 = 2400 N. At constant speed the driving force equals the resistive force.
Answer: D. Power is the rate of doing work, P = W/t, in watts (J s−1). P = Fv is derived from this definition, not the definition itself.
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