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Re: PHYS-L Digest - 3 Feb 2004 to 4 Feb 2004 (#2004-35)

On Wed, 4 Feb 2004, Automatic digest processor wrote:

Date: Tue, 3 Feb 2004 09:40:47 -0500
From: Bob Sciamanda <trebor@VELOCITY.NET>
Subject: Re: PHYS-L Digest - 1 Feb 2004 to 3 Feb 2004 (#2004-34)

I would comment that this is an instance of the weakness (incompleteness?)
of attempts to bypass the force concept and begin with (and be confined to)
energy/momentum concepts (lifted out of the air). To me, the force concept
(embodied in all three N laws) is the indispensible root basis of Newtonian

It is, of course, true that the force concept is the root basis of Newtonian
mechanics. That it is indespensible, I would argue is not true. One could,
for example, have a class (with calculus) which uses nothing but least action
principles and never mention any forces. The formulation of quantum mechanics
pushed the energy concept over the force concept as fundamental, or at least
the most useful.

I choose to omit force out the economics, given the limited time resource, and
trying to cover many diverse concepts. One has to make a decision at some
point. I could spend the entire semester on mechanics, but I don't think that
this gives a good perspective of what physics is. I could make it less
quantitative, or perhaps cover only modern physics. There are many choices,
but I don't think that force is indespensible. With just acceleration,
kinetic and potential energy I can derive Kepler's law easily (in simple
cases). I was looking to see how much I can prune. Perhaps it's not
possible to get rid of acceleration altogether, or at least not easily. If I
never *try* to do it, I will never know.

Even if you lift out of the air the conservation of gratuitously defined
energy (K + P) and momentum, how do you arrive at the existence of an
orbital trajectory without (gratuitously) adding the stipulation that the
momentum exchange is central?

From acceleration due to gravity, pointing down (towards the center of the
Earth), you can derive from geometric arguments how fast you'd have to throw a
ball to miss the ground.

It bothers me to write that the KE =3D 1/2 PE. Wouldn't it be better=
write that that KE =3D 1/2 |PE|, where PE is set to zero at infinite

absolutely. Another case of me being sloppy. :) thanks for pointing it out.

From: John Denker <jsd@AV8N.COM>
Update: Now I know there isn't one.

Consider two potentials, marked with ' and ".

' ' ' ' ' ' '
" " " " " " " " " " "
' <-- X

That is, at point X, the two potentials agree as to the slope
of the potential (i.e force), but one of the potentials has
only half the binding energy of the other. I repeat:
same force, different energy. This situation arises naturally
e.g. if you compare the Coulomb potential to the Yukawa potential.
It can be demonstrated in the classroom by rolling marbles in
suitable dishes.

I know Brother Blais wants to restrict attention the gravitational
1/r potential ... but the energy principles don't know that. To
make the result stick, at some point you will have to bring in
the *local* shape of the potential (not just its total depth)
and that is tantamount to bringing in the force.

Perhaps we should think about it this way: given a set of students who will
most likely not go into science fields, who will work most likely for
corporations, and have 1 semester to learn "Physics", what do you cover? My
feeling is that the essence of physics can be summarized by a few concepts:

* estimation, and dealing with large and small numbers
* critical thinking, and falsification of explanations
* conservation laws as a tool to describe many diverse phenomena, from the
largest scales to the smallest
* relativity, the role of measurement, and the wave nature of matter as the
"unintuitive" modern science, upon which all technology rests

Given one semester, it is *impossible* to cover every topic in a physics
textbook, so which do you choose? Given this constraint, I choose energy over
force, because I can get 90% of the descriptions correct using energy that I
can using force (even without calculus), *and* I can use it with all of the
modern physics, where I can't use force. It may turn out that Kepler's law is
a bit of a stretch without calculus and without using force, but I think
introducing force for classical physics and then not using any of it for the
modern physics is not as good a use of time as I could be doing.

I would love to hear what other approaches to this sort of class, and this
type of student population are. I try to use as few concepts as possible, in
as many ways as possible. Sometimes it doesn't work, but I keep trying to
push it in different directions, which is what motivated my original
question. I really appreciate the feedback, and would love to hear what
everyone thinks "physics" is, and what take-home message we want for
non-scientists who we feel need to have this take-home message to function
well in society. What would we like every adult in the US (or world) to know
about physics?

Brian Blais