Re: work done by friction

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

John and Chuck have both presented illustrations to show that
frictional forces do work.  John's illustration is the hand wiped
across the table top and Chuck's illustration is a box being dropped
on to a moving conveyor belt.

Yes, in both these cases frictional forces DO do work.
Does this destroy my arguments?  I don't believe so.

The standard textbook example for frictional forces doing work is that
of a block sliding on a rough surface with diminishing speed and
kinetic energy or being pushed across the surface at a steady speed:
work being done by the pushing force (not by the hand, as Jim Green
reminds us so well) while there is no increase in kinetic energy.
Note that there is relative motion between the two surfaces.  These
are the types of situations I was describing yesterday and pointing
out that, at least in the model I use, there is no work done by the
frictional force.

John's and Chuck's examples are not of this kind. These is no relative
motion between the hand and the table or the box and the belt.  If
there is slippage then the table isn't pushed over or the box is not
carried forward.  Essentially they are cases of static friction.  I
would argue that these cases are really no different in effect to a
normal contact push being applied parallel to the table's consequent
motion or the belts direction.  The mechanism of the operation of the
force is different: for the normal force the hand distorts the
surface, for the frictional force the hand "welds" to the surface.

The frictional force does do work in these no-slippage cases which are
not the typical problem of the textbook and the classroom, the
frictional force does work

John, you write: "I don't care how many experts claim it. The claim is
not correct."  I'll meet you for the situation where the is no
slippage involved; will you think again about the slippage situation?

By the way, in ALL cases, the change in the kinetic energy of the body
will "come from" the net force exerted on the centre of mass of the
body but not because work has been done.  The change in kinetic energy
can be calculated from Newton's second law; the notion of work is not
needed (and, I claim, is misleading in many situations involving
non-rigid extended bodies - which are the things the real world is
made up of).

You claim that "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."  John, your example of
the non-slippage situation does not disprove the claim fdor the common
slippage situation.

To move on:I guess again it's a question of nomenclature but I confine
my application of the term friction to solid-to-solid contacts.  Your
examples involving oils I class as lubrication and the phenomenon of
viscosity and they certainly were not included in my statements about
frictional forces (in the kinetic situation) not doing work.

In passing, you refer to my references to the ,mechanism of friction
as
"Hmmm, that's another story about friction that is often repeated but
is overstated at best."
But it is well documented in Bowden and Tabor's seminal text "Friction
and Lubrication".


Further you wrote
> 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.

In the "static" situation illustrated in your figure, the point of
application of the frictional force is moving relative to the frame in
which the energy is being evaluated.  As you rightly state, the
frictional force is not localized and does do work.  The situation I
describe is the "kinetic" case where the misleading standard textbook
diagram indicates that the frictional force is acting all over the
surface of contact and is moving with the motion of that surface.  I
claim  (with the support of the work of Bowden and Tabor) that this
diagram and the usual accompanying discussion is wrong and that the
forces are extremely localized and are not displaced during the
motion; they disappear and appear at the points of contact.

Brian McInnes