Re: positive and negative work

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Fri, 9 Nov 2001, Joe Heafner wrote:

> > From: "Waggoner, Bill" <BWaggoner@METROPO.MCCNEB.EDU>
> >
> > Assuming no change in KE of the system, I would tell a student the
> > normal force of the floor did negative work on the system. (That is if
> > we don't view the "system" as a spring that's being compressed.)
>
> NO NO NO!!!!! ARRRRRRRRRRRRRRRRRGH! The force on the student
> due to the floor does not act through a displacement, so that
> force transfers no energy into or out of the system (student).
> If it DID, then we could extract nourishment from just
> standing on the floor or leaning against a wall.
>
> Read Arons and/or Chabay and Sherwood.

This is far too parochial a viewpoint.

First of all, which of the many legitimate and useful definitions
of work are you using? Just for instance, Sherwood and Chabay,
Arons, (and Mallinckrodt and Leff for that matter) all recognize
the so-called pseudowork-kinetic energy theorem which (as Bob
Sciamanda also regularly points out) is a simple, integrated form
of Newton's second law. In this case the floor does do negative
"pseudowork" on the student in the reference frame of the floor.

Second of all, even if you do use one of the definitions of work
which involves the displacement of the point of contact, who says
the floor does not act through a displacement?  In fact, in most
reference frames the point of contact does move.  It is only in
one reference frame (i.e., that of the floor itself) that the
floor acts through zero displacement.  I can generally find
reference frames in which ANY single force does ANY amount of work
positive, negative, or zero.

Finally, as all of the above should make clear, trying to connect
"work" to "nutrition" in any situation (not just this one) is
going to be very tricky.

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