Re: work done by friction

[Brian McInnes <bmcinnes@PNC.COM.AU>]

I wrote
> > Ah, but in the model I use (as does Arons and as does Knight)
> > frictional forces do not do work.

In his reply John Denker wrote (inter alia)
> 1) The no-work claim is only valid for a particular subtype of frictional
> forces.
>
> More generally, friction between A and B does work on A if B is moving.

John, you have missed the point I was making.  I'm claiming (as does
Arons in his "Guide to Introductory Physics Teaching.") that that no
work is done by the frictional force when A and B are in contact and
are moving relative to one another and, as usually happens, a
frictional force is present.
The friction mechanism involves cold welds being made and broken and
materials being abraded away.  A pushing force would do work because
the boundary it is pushing against undergoes a displacement. The
frictional force is the sum of many, many forces that are localised in
position, appearing and disappearing at many different places as the
two bodies slide past each other.  There is no work done by the
individual forces at the weld positions (the points of real contact).
So no work is done but the first law of thermodynamics still holds.
If we take the system made up by A and B and write the first law as W
+ Q  = delta U, then the left hand side of the equation has two zero
terms in it and on the right hand side there is a book-keeping
transaction from external kinetic energy (macroscopic) to thermal
energy (microscopic) (not heat).
It seems to me that there is a problem if you want a non-zero value
for W.  Doing this explains the loss of kinetic energy but not the
emergence of thermal energy.



Widening this contribution, I call attention to the recent paper by H
Thomas Williams (AJP, 67, Aug 99, p670-680) entitled "Semantics in
teaching introductory physics".  Williams writes in the abstract, "The
large vocabulary of words we use for precise purposes in physics
contains many words which have related but potentially confusing
meanings in everyday usage. A surprising number of words we use
frequently are not used consistently in the language of introductory
textbooks."
Not surprisingly "weight" is one of the words he nominates as having a
technical use in physics that is at odds with common, everyday
definitions AND which is not used consistently within the physics
community.  Other such  words he discusses are "accelerate", "force",
"power", speed", "tension", "velocity", "dynamics", equilibrium",
"impulse", "mass", "motion", and "particle".
I think this paper is pertinent to much of our discussions.

Brian McInnes