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Re: Car acceleration



John Denker wrote:

Note that in this breakdown:
-- energy from inside
-- momentum from outside
the pseudowork behaves more like momentum transfer
than like real energy transfer. This leads me to
conjecture that pseudowork is mostly a complicated
roundabout way to talk about momentum transfer.

I *wish* I had time to give a full answer to the invitation lurking
in that statement. I'll have to content myself with a few brief
remarks for now:

1. Students need a bridge between the yawning gap between the two
parts of your breakdown. Heck, instructors need a bridge, judging by
typical postings and comments even by professionals like van
Heuvelen. I believe pseudowork can assist with this. YMMV

2. There are many interesting problems where we're interested in
mechanics questions like forces, displacements, speeds, etc which are
most easily and directly tackled using pseudowork. Can other methods
(such as momentum transfer) be applied? Sure. Are those the quickest
way and most intuitive? In a number of interesting cases, no.

3. Although most standard textbooks claim to be using "work" when
they prove and apply the work-energy theorem, what they are actually
using is pseudowork. Some texts try to get around this by avoiding
certain problems (eg. ANY problem involving friction!) or by artfully
choosing their system to avoid these issues or by invoking special
case formulas for handling say "frictional work" (often with
footnotes to AJP papers which of course every student runs out
eagerly to read). My point is: most standard texts already are using
and discussing pseudowork. They just don't call it that. But a rose
by any other name....

4. Why do texts get into work (ie. pseudowork in the mechanics
chapters) at all? Because it's a very straightforward way to
introduce kinetic and potential energy. And at a deeper level I think
it can be used as a way to introduce energy without jumping into the
subtleties of heat, internal energy, forces that don't do work, etc
before students are ready. A key idea is that at the level of point
particles (such as molecules), pseudowork is the same thing as plain
old work.

There's more to say, but I'll stop for now. Carl
--
Carl E. Mungan, Asst. Prof. of Physics 410-293-6680 (O) -3729 (F)
U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu http://physics.usna.edu/physics/faculty/mungan/