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



Wow...proofreading is tough... Got the GMm/r formula wrong... there should be no square in there.

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of William Robertson [wrobert9@ix.netcom.com]
Sent: Wednesday, December 15, 2010 4:49 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] internal/external conservative/nonconservative forces!?!?

Yes, I agree. The changes in PE are the important thing, not arbitrary
values of mgh based on arbitrary reference levels.

Bill



On Dec 15, 2010, at 3:39 PM, Bill Nettles wrote:

I agree that defining the system is important, but isn't defining
the "zero" also important. One student calculates 'mgh' and another
calculates GMm/(r(earth)+h)^2 and they get different numbers. Like
John Denker said, we don't want inappropriate uses of the
approximation, and not realizing that the zeros in the potential
energy calculation are different is an inappropriate use.

I believe what we should be emphasizing to the students who use mgh
(or U=qV) is NOT the calculation of potential energy (because mgh is
NOT gravitational potential energy), but the change in PE. Even in
the earlier "bonds breaking" criticism, it is the change in the
potential energy of the bonds that is important. Processes occur
when energy changes from potential to kinetic to _______ . Even if
there is a good argument for GPE to be absolutely (rather than
definitionally) zero as r->infinity, that's not as important as
recognizing and calculating changes in potential energy rather than
"amount." Then the 2 students above can reconcile mg*delta h and
GMm(1/r1^2 - 1/r2^2).

Problems that ask a student to calculate the potential energy of a 2
kg object 3 meters above the floor are bad!


-----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 12:14 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


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