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Re: [Phys-l] "inelastic"




----- Original Message ----- From: "John Denker" <jsd@av8n.com>

Also on philosophical and pedagogical grounds, I disapprove of
questions that hinge on legalistic analysis of the terms. It is
usually better to use a few extra words (or perhaps even diagrams)
to convey a clear picture of what is going on physically.

That's true in general, and extra double especially true when there
is less-than-complete consensus as to what the terms 'should' mean.

Let me explore this a little more. At least in all the intro texts I've used, an elastic collision is one in which both the momentum and the KE are conserved. With that 'definition' then using a term like 'partially elastic' seems to make no sense and should confuse the hell out of students. We can approximate elastic collision (air track carts, billiard balls) but recognize that there are no TRUE elastic collisions in the macroscopic world, and if I remember by particle physics correctly, true elastic scattering is all but non-existent as well. (p,p') scattering off the ground state of a nucleus (with Q = 0) probably qualifies, but that's about it. In other words, collisions are inelastic events. 'Totally' or 'perfectly' inelastic seems to be a stretch. (I usually want students to recognize a 'sticky' collision as an indication that it is inelastic but have not used a qualifier for collisions where the final KE = 0.) So I guess I second JD here in that 'totally or perfectly inelastic' would need to be carefully defined if used at all, but 'perfectly elastic' seems highly redundant to me. ;-)

Rick