[Physltest] [Phys-L] Re: Static friction and thermal energy

[Carl Mungan <mungan@USNA.EDU>]

> I read today Carl Mungan's excellent article on work-energy
> relationships (TPT January 2005). Congratulations, Carl!
> I was pleased to note that Carl acknowledged many Phys-L members.
>
> Well, I have a question on a point made in the article. Carl wrote:
>
> "Static friction does not change the thermal portion
> of the spool, in striking constrats to kinetic friction
> on a sliding object. This is *not* a consequence of
> the fact that static friction does zero particle work!"
>
> Then Carl gives two examples which support this point but he does
> not explain the reason why "static friction does not change the
> thermal portion of the spool". I see that Carl's point is clearly
> valid but I don't know why
> (I would have thought that it is because static friction does zero
> work...). What is the physical reason, then?

Dear Antti: Thank you for your kind words. I am not sure I have
anything particularly deep to add to what I wrote. But let me try.

Some forces (such as gravitational and electrostatic forces) are
conservative (ie. they are only a function of coordinates and have a
potential associated with them). As the name "conservative" implies,
these are associated with conservation of mechanical energy and do
not couple to the thermal degrees of freedom (ie. they are
nondissipative).

However, the converse* is not true: it is NOT the case that
nonconservative forces are necessarily dissipative. In addition to
static friction, other examples of nondissipative nonconservative
forces are tension and normal forces. (One might suppose that static
friction and normal forces cannot do work. This is incorrect. Think
of a box in the back of a flatbed truck in the former case, or a box
on the floor of an elevator in the latter case.)

In all fairness, it should be pointed out that these examples of
nondissipative nonconservative forces are not energy sources. An
ideal string only pulls (so that the tension does work) if some agent
on the other end is actively providing energy: a spring perhaps. The
normal force of the elevator is "powered" by the electric motor. The
flatbed truck by the gas engine.

I haven't really answered your question: How can we tell if some
given nonconservative force (which is doing positive or negative work
on some object of interest) is dissipative or not? I think this
question is quite subtle and reminds me of discussions (eg. in the
Feynman Lectures or in the lovely book "The Refrigerator and the
Universe" by the Goldsteins) about the puzzle of how irreversibility
can arise in classical physics when Newton's laws are perfectly
reversible. If you zoom in on a log burning in the fireplace, every
atomic collision can be elastic and yet the macroscopic process
clearly is one way in time. Same if you blow up on two rough objects
sliding over one another: electrostatic (conservative!) interactions
between electron shells somehow sum up to a dissipative process.
Ditto for a bunsen burner flame playing on a beaker of water. At a
microscopic level, heat is work! (This is another reason I reject the
attempt to define Q as "microscopic work". At the microscopic level,
Q is meaningless; it's a macroscopic thermodynamic quantity, and
seldom useful in irreversible thermodynamics to boot.) FWIW, Carl

*Footnote: conservative implies nondissipative, but nondissipative
does not always imply conservative.
--
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
       U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5040
mailto:mungan@usna.edu       http://usna.edu/Users/physics/mungan/
_______________________________________________
Phys-L mailing list
Phys-L@electron.physics.buffalo.edu
https://www.physics.buffalo.edu/mailman/listinfo/phys-l