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

[William Robertson <wrobert9@ix.netcom.com>]

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