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RE: Gedankening


Sorry, Joel, but I don't agree with your characterization of
Newton's first law (N1). I think that the real meaning of N1 is
"there exist inertial frames". That is not a proposition that can be
demonstrated by experiment, because it is a definition.
You could sharpen the discussion by picking a different example.

Regards,
Jack

Jack:

I think that there's a difference between "meaning" and coming up with some
notion in the first place. Many times now I've watched a classroom of
students decide that it must be "no force" which is what "maintains"a
constant velocity or "causes" a velocity to be maintained constant. Just
about every time when this is first proposed some student points out that
no "real" object on earth has NO forces on it and the discussion almost
always is turned by some class member to rockets or asteroids in space.
The group then seems to move during this discussion from "no force" meaning
NO forces to "no force" meaning something like our net force. From this
someone brings the discussion back "down to earth" by suggesting that
instead of NO force what might work is no "excess" or no "left-over" force
once all are taken into account.

This appears to be a gedanken experiment to me. They might if they pursue
the issue further and further decide on the "there exist no inertial
reference frames" as a meaning for what they first generated via gedanken,
but real people (students) can be observed spontaneously "gedankening."
For a description of the typical run of the discussion in the example
described above, see Chapter 12 in (dare I mention it?) _Constructivism:
Theory, Perspectives and Practice_, C. Fosnot (ed), Teacher's College Pr,
1996.

The secret to the quality and efficacy of gedanken experiments as far as
I'm concerned in physics teaching is that the experiments are best
generated by the students when and where they see the need.

Dewey

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Dewey I. Dykstra, Jr. Phone: (208)385-3105
Professor of Physics Dept: (208)385-3775
Department of Physics/MCF421/418 Fax: (208)385-4330
Boise State University dykstrad@bsumail.idbsu.edu
1910 University Drive Boise Highlanders
Boise, ID 83725-1570 novice piper

"Physical concepts are the free creations of the human mind and
are not, however it may seem, uniquely determined by the external
world."--A. Einstein in The Evolution of Physics with L. Infeld,
1938
"Don't mistake your watermelon for the universe." --K. Amdahl in
There Are No Electrons, 1991.
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