Re: [Phys-l] conservation versus constancy

["Bob LaMontagne" <rlamont@postoffice.providence.edu>]

Simply, what I am saying is that it is strange to use the words constant and
conserved when during the only part of the interaction that you can actually
call a collision, the KE can go to zero. On the other hand, momentum, being
the temporal effect of a force, is conserved throughout the entire
interaction - before, during, and after. KE, the spatial effect of a force,
definitely is not - either during elastic or inelastic collisions. I don't
know how you wax on to a class about KE conservation when it was zero during
the most important part of the process. I think it best not to even bring it
up until the concept of conservative forces is encountered.

Otherwise, we have two situations to present to the students. One, where we
use the incantation "elastic" and declare the KE to be the same before and
after the collision, and a second one where we just say "oops" and only use
momentum conservation along with some mumbling about the KE must have been
turned into some other form for some reason.

Slingshot:

The slingshot effect is elastic - i.e., the KE before equals the KE after.
The mass of the planet is so much more than the mass of the satellite that
one sees a visible change in the speed of the satellite but not the planet.

It's just like a bowling ball striking a stationary tennis ball. The bowling
continues on with no visible change in speed, whereas the tennis ball moves
at twice the speed of the bowling ball (a slingshot in one dimension!)

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of Spinoza321@aol.com
> Sent: Monday, October 16, 2006 12:39 PM
> To: phys-l@carnot.physics.buffalo.edu
> Subject: Re: [Phys-l] conservation versus constancy
>
>
> In a message dated 10/16/2006 11:21:16 AM Eastern Standard Time,
> rlamont@postoffice.providence.edu writes:
>
> Again,  Not to nit-pick, but:
>
> KE is neither "constant" nor "conserved". In  fact, during the actual
> elastic
> collision it can practically disappear.  It's really a matter of how much
> of
> it reappears as gross KE and how much  as microscopic KE after the contact
> phase of the collision.
>
> Bob at  PC
>
>
>
>
>
>
> By definition elastic collisions conserve KE,  inelastic collisions
> don't.
> So what are you saying here?
>
> Bob Zannelli
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