RE: Work done by Friction

["John Gastineau" <john@gastineau.org>]

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

> -----Original Message-----
> [mailto:owner-phys-l@atlantis.uwf.edu]On Behalf Of Tim Burgess

> 	I'm with a group of high school physics teachers. We are in disarray.
>
> 	Textbooks that we use commonly refer to the "work done by friction"
> in  slowing the speed of objects.  There are some who think this approach
> is flawed.  Others indicate that the right answer is commonly obtained
> by treating such friction as work.
>
> 	Some would just ignore the textbook treatment and indicate that
> "work done by friction" is not the way to address the energy "dissipated".