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

[Bill Nettles <bnettles@uu.edu>]

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





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.
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