Re: work done by friction

[John Denker <jsd@MONMOUTH.COM>]

At 04:18 PM 10/27/99 +1000, Brian McInnes wrote:

> Now you and Chuck are right in claimng that the frictional force acting
> during that short time interval does work.

Fine.

> As far as the "make-and-break
> welds" model of frioction goes, the welds formed as contact is made do not
> break in this situation.

Firstly, I don't see what the make-and-break model has to do with this.
The correctness of my rejection of the "friction doesn't do work" notion is
independent of whatever microscopic mechanisms are assumed.

Secondly, the notion of welds being made but never broken is inconsistent
with the idea of dynamic friction that we previously agreed was the central
subject of discussion.

> So, in some specialised situations (and, John, your tryamobile is sure
> specialised) frictional forces can do work but in general they do not.

Huh?  Let's see.  Conveyors are specialized.  Acceleration by wiping is
specialized.  Automobile clutches are specialized.  Brakes are specialized.
 What are we going to do, take all cases where dynamic friction does work
and declare them to be trivial specialized exceptions to the sacred rule
that "friction doesn't do work"?

> Tyhe thiung to do is not to claim that they do (as in the standard texkbook
> illustration of a block sliding to rest on a table with a rough surface)
> but to calclate the chjange in kinetic energy by using the CM equation and
> look at energy alterations by applying the full first law of thermodynamic
> to a properly chosen system (here the block and the table,

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.

Perhaps there is confusion because we have here a case of nonconservation
of work.  Ok, fine.  Work is not separately conserved.  Energy is
conserved.  Work was done on the block, and the energy was converted to
heat.  What is the problem here?

We expect dynamic friction to convert work into heat.  What else could we
possibly expect?

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