Physics · Work 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 component of the force along the displacement counts, so work is the scalar product of force and displacement. It is positive when the force helps the motion, negative when it opposes it, and zero when the force is perpendicular to the displacement. For a force that changes, the work is the area under the force-displacement graph.
In a conservative field the work done between two points does not depend on the path, and the work done round a closed path is zero. Gravity is conservative; friction is not.
Power is the rate of doing work.
Students multiply the full force by the distance when the force is at an angle. Use only the component along the motion, F cos θ, and remember that a force at 90° to the motion does no work.
Answer: Work done = 2940 J; average power = 245 W
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