Re: Introducing WORK

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

On Mon, 2 Nov 1998, Rick Tarara wrote:

> I, like Ludwik, will be starting Work and Energy today, but unlike past
> years, I have already covered momentum and collisions.  To deal with Elastic
> collisions, I needed to introduce KE, so I did so as a simple 'book-keeping'
> tool.  We have not really discussed Energy per se, but rather recognized
> that we can calculate this quantity that is named Kinetic Energy and that,
> in certain types of collisions, this quantity is conserved and is therefore
> useful in helping us predict the results of a given collision OR to measure
> certain parameters of the collision (such as the mass of a target particle.)

Rick,

I think it is better to deal with momentum first also.  But, in that case
(and *if* you are going to deal with the concept of elastic collisions at
*all*), might it not be better to introduce the "elastic colision" as one
in which the relative velocity before the collision is equal and opposite
to that after the collision?  This does require some reference frame
skills, but that's not such a bad thing and as long as you stay in one-d
it isn't too difficult either--certainly no more so than trying to use
conservation of KE.  Then, when you *get* to kinetic energy, you can point
out that an elastic collision also happens to be one that conserves
kinetic energy. 

John
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