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[Phys-L] Re: Energy is primary and fundamental???



----- Original Message -----
From: "jbellina" <jbellina@SAINTMARYS.EDU>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Thursday, August 11, 2005 8:19 AM
Subject: Re: Energy is primary and fundamental???


Back to the pedagogy.

I can think of few concepts that are more difficult to deal with than
acceleration. If an energy first, or momentum first approach would
allow one to avoid acceleration, so go back to Newton original idea
about force and momentum change, I think it would do a great service.
Of course one would have to check carefully that the sort of conceptual
understanding that John Clement calls for is indeed there.

joe

I'm also starting to wonder what the real objection to energy first is.
We're seeing some concern about whether or not energy is "fundamental", and
I find I'm not sure what is meant by that expression. To me, conservation
of energy and conservation of momentum are fundamental and necessary
concepts for physics. In my AP classes, I tell my students that if they are
unsure of how to proceed in a problem that they should consider these two
first and from there move on to free-body diagrams and Newton's Laws. As
for energy being "real" or not, does its supposed lack of "realness" mean
that the people objecting to it don't employ it in their classes? Does it
become more "real" later in the course?

I personally don't introduce energy right away, but I have colleagues who
do. I find nothing wrong with their choice, but perhaps since so many of
the negative responders seem to be coming from the college ranks, the
problem here is poor communication and definition. College courses, AP
courses, etc tend not to emphasize concepts (despite what many/most of us
may believe), but rather to emphasize calculation. I certainly agree that
starting energy calculations first, without forces, work, etc would be
pretty silly. What is NOT silly, imo, is beginning to introduce the idea of
energy transfer, energy balance, etc - The concepts, not the math. The math
can come in later. My students have no problem "getting" that energy can
transfer from one "object" to another, and that the total energy should
remain constant as long as additional objects are not interfering. They
also see that a third object interfering can lead to an increase or decrease
in the total energy of the original two objects. They further see (later)
that this can be expressed as a mathematical relationship that will allow
them to determine specific values. Now what, exactly, is the problem with
that approach WHENEVER it occurs?

I can pretty much guarantee that many college physics students who can
employ energy conservation using the equation: mgh = 1/2mv^2 have little or
no idea why that is true (or, more importantly, under what conditions it is
NOT true). In physics, the concepts are fundmentally simple to state, and
that tempts us as teachers to assume that they are, therefore, as easily
understood. We feel safe in moving quickly, or immediately, into using
mathematical expressions and performing calculations. The reality is,
however, that these simply-stated concepts are really NOT particularly well
assimilated by students, and they often have no clear idea WHY the equations
they are using WORK!

Look at Newton's 3rd Law, for example. Simple isn't it? Two objects
interact and feel equal but opposite forces. What could be simpler than
that? Now ask a student, "Suppose a rider falls off her horse and is being
dragged across the ground. Who exerts the greater force, the horse or the
rider?" We all know that a disconcertingly large number of students are
going to answer that question incorrectly. For the more physics savvy
students maybe we amend this question to have the horse and rider
accelerating, which leads even more of them astray.

Anyway, interesting discussion.