[Phys-L] Re: Energy is primary and fundamental? (was RE: First Day Activities or Demos)

[jsd at AV8N.COM (John Denker)]

rlamont wrote:
> This approach seems too much like some form of "voodoo" to me.
> Ascribing some poorly defined entity called "energy" to anything
> that is thought of as change is too close to the pseudoscientific
> babble we hear from people who believe in ESP, faith healing,
> homeopathy, etc. If you have no concrete explanation for a
> phenomena you say some form of "energy" is transferred from one
> object to another. Of course, in most cases, that energy cannot
> be seen or sensed - or measured. If I was a student just
> encountering physics, I would be very turned off on the subject
> if introduced to it with this nonspecific mumbo jumbo.

Here are my partially-baked thoughts on the topic:  It
seems to me that practically everything we do involves
a considerable amount of abstraction.

Kinematics class has to start somewhere ... but where?  If
you start with force, somebody will object that force is
an abstraction.  If you start with energy, somebody will
object that energy is an abstraction.

So then, what are the arguments on either side?  I'm guessing
now, but I imagine one sect argues that force is "somewhat"
less obscure, because people (i.e. students) can literally
get a "feel" for force using their hands.  But the other
sect argues that students can "see" a brick on a high shelf
has having more gravitational potential energy than a brick
on a low shelf.  One thing can be seen;  the other can be
felt.  The two arguments seem equally unconvincing to me.

=======

Maybe it comes down to a question of taste.  It is proverbially
unwise to argue about taste, but let me at least try to
_explain_ my taste (recognizing that yours may vary).

1) One of my hobbies is flying airplanes, and teaching people
to fly airplanes.  Very early in the pilot-training process,
my students learn to think of energy as something very real.
If it was abstract before, it isn't now.

Fighter pilots have a saying, "energy is life".  It's that
important.  In an airplane, the forces pretty much take
care of themselves, until you reach certain limits of
what the plane and/or pilot can tolerate.

But energy has to be managed.  Serious thought goes into
managing the energy in common situations.  If you have
KE (airspeed) you can turn it into PE (altitude) quite
quickly and quite efficiently, and vice versa.  But if
you have too much total KE+PE, it can be a real nuisance
to get rid of, and if you have too little KE+PE, you can
make more but only rather slowly.

My tentative pedagogical theory is that if you put
students in situations where they need to know about
force (but not energy), they will think that force is
"natural" and energy is "weird".  And vice versa ...
you can demystify energy as easily as anything else.

2) I don't consider myself a big-time fluid dynamicist,
but I keep encountering fluid situations.  Keeping track
of forces in such situations is a losing proposition.
You learn to focus on the conserved quantities:
particles, energy, momentum, entropy, et cetera.

Also in relativity, it is easier to keep track of
momentum than keep track of force.

==

I realize that not every intro physics student is going
to be a pilot, or a fluid dynamicist, or a relativist.
But still, to my taste anyway, there is something grand
and beautiful in the conservation laws.

So I'm really making two points:  the conservation laws are
  -- powerful, and
  -- beautiful.

The beauty is separate from the power.  I know of lots of
things that are powerful but really ugly.  (Hippopotami
and certain computer languages spring to mind.)

Some of the beauty (and perhaps some of the power) comes
from Noether's theorem.  Symmetries are nice.  Everybody
likes symmetries.

======================================
Brian Blais wrote:

>   I think the connection to real life is in
> the examples.  One can talk about billiards, car crashes, and asteroid
> impacts all with the same concepts.  We do an analysis of diets, looking
> at fat turned to heat.  We look at movies as well (like the bus jump in
> Speed).  I invite them to bring things in from their lives, and we pick
> them apart, again from an energy-momentum perspective.

These are good arguments, and life would be easier for
me if everyone found them convincing (because then everyone
would agree with my taste).  But objectively, alas, it seems
there are counterarguments in some cases.

For example, if you want to prevent the car crash by
braking and/or steering, you are going to run up against
limits on the force your tires can develop.  Also, the
argument that the lethality of the crash scales like
energy is a bit dodgy;  yes, it scales like momentum
times velocity, but it doesn't obey a conservation law
the way real energy does.  In fact it scales like momentum
times velocity over length, and honest-to-goodness
energy wouldn't have that length-dependence.  So I'd say
to do a good job, you need *both* energy/momentum ideas
and force ideas.

Given the choice of which to present first, my choice is
clear.  Energy is chapter 1.  Force is chapter 4.  I am
comfortable selling the idea that the conservation laws
are powerful, elegant, and beautiful.  (The force laws
are powerful, but IMHO not nearly so elegant.)