A child on a playground swing illustrates the difference between potential and kinetic energy. At A, that point of momentary motionlessness just before the downward plunge, she has only potential energy. As she starts down she develops kinetic energy. Halfway through the cycle, at B, her energy becomes completely kinetic, then gradually changes back to potential as the swing rises again. Because of friction the cycle does not continue indefinitely -- "pumping" is a requisite of the ride.

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THE ROBINSON LIBRARY
Technology.--Mechanical Engineering and Machinery.--Machines and Mechanics (General).

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