Re: work done by friction

[John Denker <jsd@MONMOUTH.COM>]

At 05:47 PM 10/25/99 +1000, Brian McInnes wrote:
> 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.

I know that's what the claim is.  I don't care how many experts claim it.

The claim is not correct.

Consider the frictional situation as illustrated in
  http://www.monmouth.com/~jsd/how/htm/4forces.html#fig_table_friction
as opposed to non-frictional situation in
  http://www.monmouth.com/~jsd/how/htm/4forces.html#fig_table_pressure

Suppose for present purposes that the table in the figure is initially at
rest.  Then I suddenly wipe my hand along the tabletop in the direction
shown in the figure.

Guess what?  The table starts to move.

Where did this kinetic energy come from?  I claim the only place it could
possibly have come from is from the frictional force which did work on the
table.

I'll say it again:  this is a situation where

> A and B are in contact and
> are moving relative to one another and, as usually happens, a
> frictional force is present.

and the claim that

> no work is done by the frictional force

is totally, clearly, inescapably false.

This disproof is independent of the microscopic nature of the frictional
interaction between the hand and the table.

=============================================

Now for some additional minor points:

> The friction mechanism involves cold welds being made and broken and
> materials being abraded away.

Hmmm, that's another story about friction that is often repeated but is
overstated at best.  That statement is only valid for a certain subtype of
friction.

By way of counterexample, the bearings in an engine depend on a layer of
sticky oil to *prevent* metal-to-metal contact.  When I do spectroscopic
analysis of the oil I do not usually find metal in the oil -- not at a
level consistent with the "cold welding" model of friction.  I'll put it
this way: if analysis ever *does* show significant metal in the oil of an
aircraft engine, the aircraft is grounded, because it usually means a
bearing or something has failed.

> A pushing force would do work because
> the boundary it is pushing against undergoes a displacement.

Agreed.  Pushing is what is illustrated in
  http://www.monmouth.com/~jsd/how/htm/4forces.html#fig_table_pressure
and we agree that's not what we normally call friction.

> The
> frictional force is the sum of many, many forces that are localised in
> position,

I'm not 1000% sure what "localized" means in this context but I strongly
suspect that (as stated in my previous note) this is the root of the
problem.  I suspect this assertion is tantamount to the groundless
assumption that the hand in figure
  http://www.monmouth.com/~jsd/how/htm/4forces.html#fig_table_friction
is not moving relative to the frame in which the energy is being evaluated.

> 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."

I agree with that!

> 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".

We might add "is" to the list :-)

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