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Re: Swartz letter in AJP (work-energy theorem)



I'll be going away this weekend, so by the time I read the digests on
Tuesday, this thread may have gone to rest. So permit me the liberty
of responding to Jim as well. He says things very simply and plainly,
so it's easy to understand and respond to him. God bless you for
that, Jim! At the same time, it does make it easier to prod and poke
at your statements. I hope you won't take it personally.

My point was that most of the list uses the same physics language as did
Swartz, so why were some on the list belittling him.

Oh dear. Terminology is of course an issue. But I think if we're
going to communicate, we need to tolerate some differences of opinion
on the best way to define and use words. But I don't think that means
that "language" is really the issue. I try hard to understand what
someone (like Swartz) is saying from *his* point of view.
Nevertheless, after doing so, I believe I see some limitations to
presenting material a certain way, regardless of how you define key
words. It's simply too constraining. (For example, some folks want to
sweep away the frictional problem we've been discussing by *always*
choosing the system to include both the widget and ice. But by gum, I
may really be interested in the widget, in whether it's going to melt
or something!)

If the widget slows it loses energy,

I'm very unhappy with this; it's too brief. The danger is that it
becomes generalized, whereas it only happens to be true in this
particular example. If you said, it loses "bulk kinetic energy" then
who could disagree? But energy overall is another matter, because the
widget not only slows down, it also warms up. (Yes I agree that in
this example, the thermal energy gained is less than the kinetic
energy lost. But in another case it might not. A common example is a
symmetric head-on inelastic collision between two identical balls of
putty. Admittedly a somewhat artificial example.)

I consider the issue of whether it is the force or the ice which does the
work somewhat spurious.

I think that for many educators, Swartz included, this is a *key*
issue and not trivial or spurious at all. If (positive) work is done
on system A, folks (rightly) want to be able to say that A's gain is
someone else's loss. So if the widget loses energy, something else
(some other systems B, C, etc) gained the same net amount. What are
B, C, etc? We want to be explicit about this in order to properly
teach the idea of *conservation* of energy to our students. Just
saying there must be some B, C, etc is too abstract. We want to go
over as many common, concrete examples as possible. Carl
--
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/