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View Full Version : Re: POSITIVE work in cyclic motion



John Kenneth Smith
09-05-1995, 04:11 AM
Craig,
Net worked performed by a "machine" is not a function of the
beginning and final positions (be that of water, people, apples, or
pistons). I think you should get your hands on a good thermodynamics
text and read-up on "efficiency". It was to describe systems such as
this that the concept of efficiency was developed to accound for the lost
work performed. Just because a system ends up in the same configuration
that it began, does not mean the net work done by the system was zero.
What you say with the swimmer is analogous to me walking across town and
back again. Are you saying I performed no work in doing so simply
because I ended up where I began? Does a fan in a closed room do no
work? I would sure like to know where all that electricity is going if
it's not going towards work -- maybe I should call the electric company
and ask for a refund --- will you back me up on this?
Yes Craig, there is work done, unfortunately, due to friction
etc, the air goes back to its original state of inertia and the fan must
continue to turn to keep the air flowing, as the swimmer must keep
"stroking?" to keep the water moving about him/her. The swimmer does
some work, but then the friction between water molecules and
quasi-equilibrium effects cause the water to loose kinematic energy, so
he/she has to continually add work.

Thanks
John Smith