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.