Re: Action - Reaction Demo

["Dewey Dykstra, Jr." <dykstrad@bsumail.idbsu.edu>]

> I certainly can't see the forces
> the fingers exert, which we suppose are equal and opposite, and the net
> force of each finger on the putty is directed toward the other finger. How
> does the putty extrusion convince me the forces were equal and opposite?
> What if the forces weren't equal and opposite (!), could that conceivably
> make a different result?
....
> The situation of this demo is far too complicated for me to get a concept
> out of it.
>
> 	-- Donald
>
> .....................................................................
> Dr. Donald E. Simanek

Not to defend or to criticize the demo in question (squeezing the ball of
clay) or Donald's analysis of this demo specifically...

It seems to me that we *never* "see" the forces.  We never measure them
directly in any other way either.  We always measure either a deformation
or a motion and infer force from that.  Hence, the existence of forces and
their relationship to deformations, motions and to each other are all part
of a construct we call force.  We rarely "measure" even by these indirect
means all the forces acting in a situation, indeed it is probably
impossible to do so in the case of forces on moving macroscopic objects.
We generally infer the "unmeasured" force(s) by invoking our belief about
the sum of the forces for the specific situation.  So, while we can
demonstrate by experiment that the outcome is consistent with our
construction of the notion of force, it appears that we cannot demonstrate
experimentally by "measuring" a deformation associated with *each* of the
forces (since the motion is only associated with the sum of the forces)
that the sum of the forces on an object moving with constant velocity is
zero, for example.

If we run students through several iterations (jr hi, HS, intro college,
upper div undergrad) of a process which fails to make this distinction,
then is it any surprise that grad students show little tendency to be
independent of authority or canon?  Should the findings of Tobias' report
"They're not dumb..." be any surprise?

Dewey

(Please note the new exchange "426" in the phone numbers.)
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Dewey I. Dykstra, Jr.                   Phone: (208)426-3105
Professor of Physics                    Dept:  (208)426-3775
Department of Physics/MCF421/418        Fax: (208)426-4330
Boise State University                  dykstrad@bsumail.idbsu.edu
1910 University Drive                   Boise Highlanders
Boise, ID 83725-1570                    novice piper: GHB, Uillean

"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.
"Every [person's] world picture is and always remains a construct
of [their] mind and cannot be proved to have any other existence."
--E. Schrodinger in Mind and Matter, 1958.
"Don't mistake your watermelon for the universe."  --K. Amdahl in
There Are No Electrons, 1991.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++