Re: work done by friction

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

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