In article , Gary Coffman says...

That's not what the definition of work says, though.

Oh now I see your point. The pendulum trades energy between
kinetic and potential. When it is speeding up one could say
that the gravitational force is doing 'work' on the ball.
When slowing down, the ball gives energy back into the
gravitational potential - 'negative work.'

That's right, in a perfect system that's all lossless,
but the term 'work' still applies.

Jim

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