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Re: work done by friction



At 07:04 AM 10/27/99 -0800, John Mallinckrodt wrote:

you can't have it both ways. Either friction does work on
both the table and the block or it does no work at all.

That's a joke, right? Ha ha ha.

For those who aren't in on the joke, I'll say it again:

Friction does negative work on the sliding block. Friction does zero work
on the stationary table. Energy is conserved. Work is not separately
conserved. The "missing" work shows up as heat. What else could we
possibly expect?

(I can give you
definitions of work and/or find reference frames that will make it go
either way.)

I was careful to stipulate that I was evaluating the energy in the frame
where the table was stationary.

I also warned (Mon, 25 Oct 1999 23:48:49 -0500) that analyzing things in
the frame of the block requires a modicum of care and sophistication,
because it is an accelerated reference frame.

The definitions I am using, and indeed the whole notion of work done by
friction, have been standard throughout physics since at least the time of
James Prescott Joule.

The only mechanism available to effect any significant short term change
of the block's energy is friction. I don't care if you want to call that
frictional work or frictional heating, but you've made it very clear that
you want to call it frictional work in the case of the block.

Actually the block gets both frictional work *and* frictional heating.

In *any*
event it is that frictional interaction that is responsible for any
significant, short term change of the block's energy and, if the block
loses energy by that frictional mechanism, the table gains it by the
*same* frictional mechanism.

In this sentence you say that energy is conserved. So this sentence is
correct. It does not however support your opening statement which suggests
that *work* should be conserved. The mechanism in this case turns work
into heat, and partitions the heat between the various participants.
Energy (including work and heat) is conserved. Work is not conserved.

Yes, there will undoubtedly be some purely *thermal conduction* processes
that will operate over the longer term that will also transfer energy from
one system to the other but these can go either wasy depending upon
unknown details of the scenario including whether the block was initially
warmer or colder than the table.

We agree that thermal conduction is a side issue. It will quantitatively
affect how the heat is partitioned between the block and the table, but it
will not affect the fact that friction does negative work on the sliding
block and does zero work on the stationary table.

______________________________________________________________
copyright (C) 1999 John S. Denker jsd@monmouth.com