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[Phys-L] Re: Momentum Agina



Larry Woolf wrote:
It seems that the energy change in an inelastic collision
should not depend on the frame of reference, since the
energy change will increase the temperature of the objects.
The temperature change should be independent of the frame
of reference.

This is the point I tried to make earlier. We shouldn't be asking "why
is it that 1/2 of the KE is lost regardless of the mechanism." Instead
we should be asking "why is a PARTICULAR amount of KE lost regardless of
the mechanism."

Bernard Cleyet wrote:

I think Dan has the answer -- I'll add one more obvious item.

Very simply KE is dissipated until both are moving at the
same speed whether it's thru gross inelasticity (one ball is
sticky putty) or it's plastic (or not) deformation of a steel
railroad car coupling, or whether it's completed slowly or quickly.

This is the key and I don't think it is obvious. KE is dissipated until
it is no longer necessary to dissipate it.

Consider, for example, a ball rolling up a ramp. If it starts with KE =
10 J, it rolls up until it loses 10 J of KE, regardless of whether there
is friction, air resistance, etc.

Now, if we view the entire affair from a moving truck such that the KE
of the ball (in the moving frame) is 20 J, it still rolls up until it
loses 10 J of KE. I can imagine people on the truck asking "why is it
that 1/2 of the KE is lost, regardless of the mechanism used to stop it
from moving upward?" -- after all, it seems as though 100% of the KE
could be conceivably lost if only the mechanism was more "efficient".
This seems to be the question that David Abineri's students seem to be
asking.

____________________________________________________
Robert Cohen, Chair, Department of Physics
East Stroudsburg University; E. Stroudsburg, PA 18301
570-422-3428; www.esu.edu/~bbq
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