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Re: Work/Energy theorem ?



[I replied to this note yesterday and never saw it distributed so I am
taking the liberty of revising and reposting it. Apologies if it was
distributed already to some on the list.]

On Sat, 15 Mar 1997, Dario Moreno wrote:

Charles in riding a train and watching the outside panorama.
He notices a specially beautiful tree and been a student of
physics, thinks: "that tree has kinetic energy".

After a while, the train stops and looking the same tree again,
Charles thinks: "Now the tree is at rest ! Since there are no
changes in the potential energy of the tree, where dit it's
kinetic energy go?"

This question may have been intended and certainly seems to have been
taken by others as some kind of argument against taking work-energy
theorems too literally or applying them in "improper" reference frames. I
don't agree and see no reason to think that the laws of physics change or
are invalid in some reference frames. After all, a greater mind than mine
took this point of view and the general theory of relativity was the
result.

Since Charles is a "student of physics," he shouldn't have any trouble
figuring out what happened. After all there was a period of time when a
uniform "gravitational" field in the "forward" direction was clearly
acting on everything in Charles' view including himself. Indeed, he
personally experienced this gravitational field as his seatbelt struggled
to keep him from sliding off of his seat and as his coffee slid "forward"
off his tray and onto his knees. He should have no trouble calculating
that the "forward" force on the tree due to this field did negative work
as the tree moved "backward" and in an amount exactly equal in magnitude
to the initial kinetic energy of the tree. The laws of physics,
therefore, *require* that the tree come to rest and Charles should be
*outraged* to see any other result.

Now I admit that I wouldn't want to take this approach with introductory
students, but as accomplished "students of physics," I think that we ought
to be perfectly comfortable with this viewpoint.

John
----------------------------------------------------------------
A. John Mallinckrodt email: mallinckrodt@csupomona.edu
Professor of Physics voice: 909-869-4054
Cal Poly Pomona fax: 909-869-5090
Pomona, CA 91768 office: Building 8, Room 223
web: http://www.sci.csupomona.edu/~mallinckrodt/