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Re: COLLISION



At 09:37 AM 11/26/99 -0500, David Abineri wrote:
>When a moving railroad car collides with a stationary car of equal mass
>elastically, the one car stops and the other begins moving with the same
>velocity as the first.
>
>If they now collide inelastically by coupling together, half of the
>kinetic energy of the system is lost.
>
>Now, my concern is, where has the kinetic energy 'gone'?

Good question. See below.

>Certainly air
>was moved, sound was created and deformations took place but surely all
>this happened in the elastic collision too.

Good point. These are nonidealities which can be neglected if we handle
the main effects correctly.

To refine the experiment:
a) In the case where the collision is supposed to be elastic, let's make
it more elastic by replacing the hard "bang" collision by something
involving a nice springy spring. This will greatly reduce the nonideal
deformations, sounds, et cetera.

b) In the case where a coupling is desired, use a spring with a shock
absorber (and a coupling clip, so cars stay together while the oscillations
are dying down). Again, this will greatly reduce the nonideal deformations,
sounds, et cetera. The no-longer-kinetic energy will appear as thermal
energy in the shock absorber.

Bottom line: If done correctly, the analysis is pretty much the same in
both cases. An interesting intermediate case is the almost-elastic
collision (use a spring, no coupling clip, and a wimpy shock absorber).