Re: ENERGY BEFORE Q

["RAUBER, JOEL" <JOEL_RAUBER@SDSTATE.EDU>]

The way the Physics 2000 text deals with spring potential energy and energy
topics is in this order:

1) Kinetic Energy
2) Gravitational potential energy
3) Work
4) Potential energy in springs.

So that author hasn't found a simple way to present the topic of spring
potential energy without first presenting the work concept.

Joel

> -----Original Message-----
> From: Ludwik Kowalski [mailto:kowalskiL@MAIL.MONTCLAIR.EDU]
> Sent: Friday, October 12, 2001 3:03 PM
> To: PHYS-L@lists.nau.edu
> Subject: Re: ENERGY BEFORE Q
>
>
> I agree with Bob (see below) that the impulse-momentum
> theorem, and the conservation of p, may help prepare
> students for dealing with the idea of conservation of another
> quantity E=KE+PEgrv+PEspr, in the idealized world of
> two conservative forces.
>
> But can this theorem be used to show (without leaning on the
> concept of work) that PEspr=0.5*k*x^2? I was not able to do
> so through simple algebraic manipulations. Can somebody
> show how to prove that PEspr=0.5*k*x^2?
>
> We all know how simple the proof is when the idea "PEspr is
> the work required to compress the spring" is used together with
> the experimental force law (Hooke). Work is the average F,
> which is k*x/2, times the distance, x. Thus, PEspr=0.5*k*x^2.
> Without work the proof seems to be more difficult. If we agree
> on this then we have a good argument against the "do not lean
> on the concept of work" suggestion.
> Ludwik Kowalski
>
> Bob Sciamanda wrote:
>
> > My approach to much of this has been be-labored in past incarnations
> > of this thread.  Let me this time add that an approach
> which I have often
> > (fruitfully) taken is to immediately follow Newton's laws with the
> > impulse-momentum theorem and the consequent conservation of linear
> > momentum  (this is "naturally" motivated by the students'
> queries about
> > N3).
> > After this exploration of the implications of the TIME
> integral of F=ma,
> > it can be made quite "natural" to investigate the SPACE
> integral of N2.
> > Thence to the work-energy theorem, conservative forces . . . etc.
> >
> > My motivation is to lift as little as possible "out of the
> air", but to
> > rather explore in a somewhat "natural" manner.  But at the
> same time,
> > all of this is preceded by the admission that our intuition
> is not really
> > this clever . . . we are using the hindsight provided by the giants
> > upon whose shoulders we teeter.