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Re: work done by friction



As in other instances of these "work-energy" threads, I stress that one
should distinguish between the two very different questions:

1) Does a given force do work? and
2) Is the mechanism or agent represented by a given force a source or sink
of energy?

Eg.: When I push myself away from a wall, the force from the wall does
work (defined as its line integral over the displacement of my CM), but
the wall is not an energy source. The work of the wall source is,
however, a numerical measure of the kinetic energy released by internal
muscle forces.

Confusion arises because when the word "work" is used as defined in the
First Law of Thermodynamics, the answers to the above questions 1) and 2)
are necessarily correlated; but when "work" is defined in Mechanics simply
as "the line integral of a force over the CM displacement of a system"
there is no necessary connection between the two answers - even though
there is always a numerical equality between the mechanical work done on a
system and the change in the CM kinetic energy of the system.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

"Clearly there is no room for disagreement about simple mathematics. But
there may be disagreement about the physical significance of it."
- J.S. Bell, Speakable and Unspeakable in Quantum Mechanics.