Re: Why work before energy in texts

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Mon, 15 Oct 2001, Carl E. Mungan wrote:

> Next question. At the macroscopic level, every force except gravity
> and springs (and electric next semester) is nonconservative.
>
> Yet at the microscopic level, all of these nonconservative forces
> (kinetic and static friction, tension, normal force, drag force,
> applied force, etc) are all conservative. (After all, they are just
> electric forces.) I think this is what enables me to bridge the
> work-energy theorem (which is about the pseudowork) with the first
> law of thermodynamics (which is about the real work).
> IS THIS CORRECT? JOHN M, BOB S -> help?

This is essentially how I see it except that I'm not completely
sure what you mean by "real work".  I tend to equate (but I am not
dogmatic about equating) thermodynamic work with what I call
(admittedly idiosyncratically) "system specific external work."
This is the work done by all external forces considering the
motion of their points of application with respect to the system
center of mass and should be contrasted with what I call (again
idiosyncratically) "frame specific external work" which is the
work done by all external forces considering the motion of their
points of application in a chosen inertial frame.

For example, when I push on one side of a balloon, I do both
pseudowork and frame specific external work.  However, I do very
little if any system specific external work.  The result is that
the bulk KE of the balloon increases, but its internal energy does
not change (very much.)

In my opinion thermodynamic work is a slippery beast that changes
definition to suit the circumstances.  This is because its
definition depends on what forms of energy you want to consider
"internal."  In the balloon example, most people would not
consider its bulk kinetic energy to be "internal" work that merely
alters the bulk kinetic energy is not thermodynamic work.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm