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Re: Introducing WORK



At 5:13 PM -0500 11/2/98, Raacc@aol.com wrote:
In a message dated 11/2/98 2:08:23 PM Central Standard Time,
ajmallinckro@CSUPomona.Edu writes:

<< 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.

Rick and John,

I too cover momentum before force and would be interested in hearing your
comments on this approach. One problem is I have not found a textbook that
covers momentum first. So, I have to jump around in the text. I do make it
clear which sections will be covered at the beginning of the semester, but
still get students resisting this approach. My approach is very visual and
graphical and I find students wanting an equation for every problem rather
than doing some simple graphical analysis and modeling to see what works.

John,

At our school, we are doing 'Momentum First' with the 'General' class (not
with the more traditional Cutnell & Johnson course. The 'General' class
uses Zitzewitz (Glencoe) and we DO have to 'jump around'. Once the kids can
find the Center of Mass velocity of two objects, they have to focus on EACH
object and use the reasonable assumption (observation) that the speed of
approach is equal to the speed of separation. This will later become
KEinitial = KEfinal.

We also do some 'partially elastic' collisions where the ratio of Vin/Vout
= coefficient of restitution. I don't like term (jargon) of Coefficient of
Restitution. We just refer to a '50%' collision, where the incoming speed
is twice the outgoing speed. (Later this will be looked at in terms of KE
lost).

Most of the kids do well in learning the 'algorithm' of such collisions and
I REFUSE to 'give the equations' for Relative Velocities. They MUST wave
their hands around enough until they can figure it out. Otherwise it's too
easy for them to slip up on a minus sign or such.

Most are quite appy to 'think it through' and SOME (a FEW) will come up
their own equations and manage to keep things straight.

Newton's Laws come SO easily, once they appreciate the conservation of
momentum!

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britton@odie.ncssm.edu you have forgotten everything
North Carolina School of Science & Math you learned in school.
(919) 286-3366 x224 Albert Einstein, 1936