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Re: [Phys-l] internal/external conservative/nonconservative forces!?!?



But one more thing -- I just realized that I do use the words "external force", but not as a word that characterizes a specific type of force, but rather to indicate that it is being applied by an agent that is not part of the system currently under consideration. So ANY force can be external or internal depending on the choice of system. Obviously, this has NOTHING at all to do with whether the force is conservative or not.

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Philip Keller
Sent: Wednesday, December 15, 2010 8:54 AM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] internal/external conservative/nonconservative
forces!?!?

I don't see why this is an issue. "Energy" is an abstract quantity, not a
thing-in-the-world. You can say it "resides" wherever you like. Can you
design an experiment that favors one view over another? Or a problem that can
be solved if you hold one of those beliefs but not the other? This seems to
me to be philosophy or psychology, not physics. [Not to bring this issue up
again, but it also seems like philosophy/psychology when we debate whether the
force causes the acceleration of the acceleration causes the force.]

As for the earlier question: I think I know why it is important to
distinguish between conservative and non-conservative forces, and think I can
design an experiment to see if a force is conservative. But I have no
understanding at all of the terms "internal" and "external". I'd never seen
them before this thread started and I can't imagine teaching them. I don't
even make a big deal about "contact" vs. "non-contact" forces -- in the end,
aren't all forces "at a distance"?

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of William Robertson
Sent: Wednesday, December 15, 2010 1:14 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] internal/external conservative/nonconservative
forces!?!?

Stating that the energy properly ascribed to a system resides instead
in one object of the system is not an approximation. Rather, it is a
small lie. Small lies are fine as long as we explain to students what
the lie is, and why it's okay to proceed with the small lie. And here
I am not talking about using mgh rather than the universal law of
gravitation.

Bill





On Dec 14, 2010, at 5:51 PM, John Denker wrote:

On 12/14/2010 05:19 PM, William Robertson wrote:
I believe that the concept of system is given short shrift in
too many physics or other science texts. There exists research
showing
that understanding or not understanding a choice of system can
dramatically affect one's problem solving ability in physics.

Well, that cuts both ways.

Yes, physics is about principles. But physics is also about
applications
and approximations.

In the physics course, I want students to learn the principles. But
just as importantly, I want them to learn how to make well-controlled
approximations.

If we are talking about the earth/moon system, the gravitational
energy
is clearly in the system, not "in" the moon. The principles of the
thing
are clear, and the same principles apply -- in principle -- to every
other
gravitating system.

On the other hand, in a very wide range of practical applications,
including soccer balls, planes, trains, and automobiles, we find "m
g h"
is an exceedingly good approximation, and is significantly simpler
than
"G M m / r". Treating the earth (and the earth's gravitational field)
as imperturbable is an approximation. Like all approximations,
sometimes
it is appropriate and sometimes it is not.

Deciding what approximation to use in this-or-that situation requires
judgment and skill.

It is ultra-super-important that students understand we are not peeved
about the approximation but rather about certain _inappropriate uses_
of the approximation.
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l