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



Could you elaborate on this? If I read you correctly, then it's okay to say that the kinetic energy of a moving object resides somewhere besides in the object. It would also seem that all talk of energy within a system, and energy input to and output from a system, is meaningless. Maybe I'm misunderstanding you.

Regarding an object and the Earth, I can give you a thought experiment that shows the gravitational potential energy does not reside solely in the object. Simply remove the Earth. No Earth, no more potential energy. If the energy were associated only with the object, wouldn't the object still have that energy when I removed the Earth? Or you could remove the object rather than the Earth. The energy again goes away. It only seems logical to me that the energy is contained in the system of the object and the Earth.


Bill




On Dec 15, 2010, at 6:54 AM, Philip Keller wrote:

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