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,
I question your statement:
<<Then, when you *get* to kinetic energy, you can point
out that an elastic collision also happens to be one that conserves
kinetic energy. >>
First, I don't know if there is such an animal that can be called an elastic
collision. But in the ideal situation, assume one does exist. Still, I would
agree that KE before the collision and KE after the collision are the same,
but disagree that KE is conserved. Consider two equal masses with equal and
opposite velocities in a head-on elastic collision. What happens to KE at the
instant they collide? What will a plot of KE versus time look like?