MDCAT Physics · Physics · Work, power and energy. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Work is done when a force moves its point of application. Only the part of the force along the displacement counts, so W = Fd cos θ. A force at right angles to the motion does no work, and a force that opposes the motion, such as friction, does negative work.
If the force changes during the motion, the work done is the area under the force–displacement graph. A stretched spring is the usual case: the graph is a triangle.
Power is the rate of doing work. A machine never gives out as much useful energy as it takes in, and efficiency tells us what fraction is useful.
Students divide the wrong way round in efficiency questions. Efficiency is output ÷ input, so the input power is always the larger one: input = output ÷ efficiency.
Answer: Useful power 6.0 kW; efficiency 60%.
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