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



Brian Blais wrote:

.... I tried to answer the question, "what is the minimum
number of concepts necessary to teach the most physics". If I
introduce, say, 2 concepts and use them in 20 different contexts, then
the students may come to understand the power of these methods. I
found, empirically, that position, then velocity, then energy, then
momentum carries me very very far. I might not be able to do *every*
example, or even some people favorite examples, but I can do quite a bit
and I think the message comes through.

Yessssss!!!!!!

============

I thought of another reason for emphasizing energy:
CHEMISTRY.

First of all, energy is important to chemists. (Force is
incomparably less useful.)

Secondly, the idea of conservation laws in general is important.
What chemists call "balancing the reaction equation" is really
just an exercise in applying a bunch of conservation laws.
Chemical reactions conserve each atomic species separately
(which is really just the definition of chemical -- as opposed
to nuclear -- reaction) ... so there are ~92 separate conservation
laws. Throw in conservation of charge and you've covered redox
reactions as well.

Recently I was talking with a friend -- a PhD mathematician --
who is terribly worried about greenhouse gasses. He wanted to
get rid of CO2 somehow, perhaps by burning it. It took him a
while to get his head around the idea of carbon as a conserved
quantity; no matter what you do the number of carbon atoms
isn't going to change. The point of my story is that from a
public-policy point of view, I want to live in a world where
the public has some understanding of how strictly conserved
these things are.

======

Another item: Note that the Newton's cradle toy illustrates energy
and momentum; despite its name, analyzing it in terms of Newton's
force laws would be a colossal pain. (Conservation of energy was
not known in Newton's day.)

Yet another: If you drop a superball, it will bounce with a good
coefficient of restitution. If you make a stack, with a big
superball on the bottom and a little one on top, and drop that,
the little one goes flying, because it bounces in the moving
frame created by the big one. If you make a "snowman" i.e. a
stack of three (big, medium, little) and drop that, the "head"
reeeeally goes flying. Entertaining demo.

Energy in a frame moving relative to the lab frame also shows
up when a baseball "bounces" off a moving bat.

Also: The energy industry, and depletion of fossil energy reserves.
http://www.av8n.com/physics/fossil-resources.htm


========================


Let me recap some of the items mentioned in this thread, things
that are most easily analyzed in terms of the great conservation
laws (conservation of energy, and perhaps conservation of momentum
also), in preference to the force laws that are traditionally
emphasized (IMHO overemphasized):
-- swinging ball demo
-- playground swingsets, pendulum clocks
-- mountain biking, BMX
-- skateparks
-- asymmetric yet balanced teeter-totter
-- chemistry
-- aircraft energy management
-- billiards
-- Newton's cradle
-- stacked superballs ("snowman"), batted ball
-- car crashes (sort of)
-- asteroid impact damage
-- energy industry, depletion of fossil energy reserves
++ conservation laws in general
++ conservation <--> symmetry


R. McDermott wrote in part:

Anyway, interesting discussion.

Indeed.