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Re: work done by friction



On Wed, 27 Oct 1999, John Denker wrote:

I reject for the Nth time that frictional forces don't do work. They do
work even in the case of a block sliding to rest on a stationary horizontal
table. At time T=0 the block has kinetic energy. At a later time it has
less kinetic energy, same potential energy, somewhat more thermal energy,
and less total energy (since the gain in thermal energy does not, except in
extraordinarily implausible scenarios, fully compensate for the loss of
kinetic energy).

So where does the lost total energy go? Something must have done negative
work on the block. That something is called friction. Of course the block
did not to work on the table; nothing is going to do work on the table
since it is assumed stationary.

Although I haven't been following this thread carefully, I think I've
agreed with most everything John D. has said up until now. However, there
is a logical contradiction lurking above. John notes that, except in
extraordinarily implausible scenarios, something *must* do negative work
on the block since it has lost energy. I would point out that, except
under equally extrordinarily implausible scenarios, the table gains energy
so, using the same logic, something *must* do positive work on it.

I don't want needlessly to prolong this debate, but for those who may not
have been around during any of its dozen or so previous incarnations on
phys-l, I will once again point out that there are at least seven
different, and useful definitions of work-like quantitities. Many
(including all that have been implicitly discussed in this thread) depend
on one's reference frame. See "All About Work," Am. J. Phys., V60,
356-365, (1992).

John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm