Re: Questions you love/hate to have asked

[palmer@sfu.ca (Leigh Palmer)]

> Seeing conservation of momentum and energy as consequences of the
> basic symmetries of nature is the truly elegant way to regard
> mechanics, but probably a bit sophisticated for young students? At
> that level, I usually discuss energy and momentum as book-keeping
> devices to keep track of work and impulse.  How do you feel about
> that approach, or do you think that the concept of force (see another
> ongoing thread!) is too vague at this level to sustain this approach?

I think the bookkeeping metaphor is excellent, fully as good as the
one I suggested. More on that later. I don't go into Noether's theorem
(a PC thing to do nowadays, and thoroughly useless in teaching concepts
in my view) when I discuss symmetry. The remark is an incentive to the
interested student to explore deeper later on. We must remember in the
introductory courses that not all of our students will go on to grad
school in physics, but we must also remember that some of them will,
and they do deserve attention to their needs. Still, I spend no more
than a couple of minutes on alluding to the symmetries of Nature, but
I do point out what they are.

The bookkeeping analogy can be carried a bit further. No one can learn
his net worth by merely looking in his bankbook. There are various
equities to be appraised and added, and how about his share of the
national debt? It's all relative; the only things that are absolute
are the *differences* encountered when money is exchanged by members
of the system, and they must balance.

I applaud your metaphor; matter of fact, I'll steal it and use it
myself from now on!

Leigh