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

[John Denker <jsd@av8n.com>]

On 12/15/2010 04:46 PM, Philip Keller wrote:
> Overall, I find this thread frustrating.  There are two different
> issues to face.  We can argue pedagogy (which is fine and completely
> appropriate for this forum) but I'd like to understand the physics
> first and then debate how to teach it.  Completely regardless of what
> approach may or may not improve my students' paradigm...I'd like to
> know for myself: is there a  well-founded physics notion of where the
> potential energy resides?  What experiment reveals this?
>
> I don't feel qualified to make pedagogical choices about material
> that I do not understand.  I think I know what potential energy is
> and how to apply conservation of energy to the standard collection of
> problems in a 1st year physics class.  But if a student asks me "OK,
> where IS the potential energy?"  I would not see the point of the
> question or why it matters to know such a thing.  I would say that it
> is not  a thing -- don't go looking for it.  And I would say that it
> has no possible affect on reality if you choose to think it is in the
> object or in the system.  But apparently, others here say I am wrong.
> I'm open to that and ready to be educated (and not for the first
> time) by the members here.  But  I am looking for an answer appeals
> to physics rather than pedagogy as its authority.

That's all well said.  I particularly like the part about not
getting dogmatic about the pedagogy unless/until one understands
the physics.

I would say to the student:  
   That is an excellent question.  The fact that nobody knows the 
   answer does not detract from the merit of the question.  Richard 
   Feynman thought it was worth his time to attack this question.  
   Let's be clear:  He tried and failed.  So anybody else who fails 
   to answer this question is in good company.  You should keep 
   asking the question.  Maybe eventually somebody -- maybe you -- 
   will figure it out.

   More-or-less everybody believes the energy resides in the field
   "somewhere" but quantifying it is tricky.  Research papers have
   been published on this topic within the last year.

   The issue matters a great deal in principle, especially for special 
   relativity and double-especially for general relativity.  However, 
   in this course we are not going to do either of those things, so we
   are going to say "hypotheses non fingo".  That is to say, we don't
   know, and /for present purposes/ we don't need to know.  For present
   purposes, it suffices to know that E = m h g to a good approximation,
   and E = G M m / r to an even better approximation.

   The rule is:  The laws of physics must say what happens.  They may 
   or may not say how it happens.  The fundamental laws rarely (if ever)
   say why it happens.  This is one of those cases where we do not know 
   how it happens.  We would dearly love to know, but we don't.

It is interesting that a student can ask a perfectly precise question
that nobody knows how to answer.

So ... you come by your frustration honestly.  A little frustration 
adds spice to life.  It's one step removed from the mother of invention.  
Maybe it's the aunt.