In article , Dave Baker says...

I'm not convinced this recent line of argument about zero net work is actually
valid in the first place. All a pendulum does is convert potential energy into
kinetic and vice versa. The only work that is done is against the friction in
the pivot and that is a scalar quantity and irreversible.

That's right. Of course there is some tiny amount of energy lost
in the knife edge pivots or whatnot. Also in air friction for
example. So real examples have some way of adding that tiny
extra energy back into the system.

Gary's example of course has the provision that 'the thing
runs in vacuum, and has perfect, non-lossy pivots.' Or
something to that effect.

Jim

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