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RE: Work done by Friction



This is a good area to question.

There was a paper by Bruce Sherwood a few years back in AJP about friction and
the conservation of energy. I am looking for the reference, but haven't yet
found it.

The short of his point is this: consider a block that slides a distance d.
Frictional force F does work Fd on the block, by the usual model. That reduces
the kinetic energy appropriately. However, that is obviously impossible. (yes,
he is saying that most textbooks are wildly wrong here.) It's impossible since
the block and the table are now warmer than they were before. The thermal
energy has to be accounted for. The resolution is in the distance d--if the
block slides a distance d, the friction force actually acted over a slightly
shorter distance due to the deformation of the surfaces.

I have waxed enthusiastic about the Chabay/Sherwood E&M text Electric and
Magnetic Interactions before on this list. They have a new book at the
classroom testing stage that addresses mechanics and thermo together, since,
they maintain, it's impossible to teach one without the other and be
self-consistent.

I have not read the book in detail, but so far it looks pretty good. It's
called Matter and Interactions, and will probably be published by Wiley.

JEG


Howdy,

I guess I don't uderstand the problem with considering friction an external
NON-CONSERVATIVE force on the system. Notice, assuming it does work and
that it is non-conservative (i.e. the work done by the force along a closed
path is NOT zero) doe NOT assume it is constant, etc.

The Work Energy Theorem follows directly from Newton's laws and can always
be pur into the form E1 + W(NC) = E2 where E1 is the total MECHANICAL
energy (Kinetic + Potential(for conservative forces)) of the system at
point 1, E2 is the corresponding MECHANICAL energy of the system at point 2
and W(NC) is the net work done by all of the Non-Conservative Forces along
the path from point 1 to point 2. The form of the Work Energy Theorem does
NOT need the concept of heat energy, chemical energy or anything else to
work properly.

It is only when you generalize the concept of Energy into the First Law of
Thermodynamics that these need to be introduced.

I must be missing something here!?

Good Luck,

Herb Schulz
(herbs@interaccess.com)