I'm entering this late, and am not sure I have read all the previous
messages in this thread, so forgive me if what I say has already been
discussed.
The distinctions I am aware of, and discuss with students, are...
(1) Elastic: kinetic energy is conserved.
(2) Inelastic: kinetic energy is not conserved.
(3) Perfectly Inelastic: the minimum amount of kinetic energy is
conserved (also sometimes stated: the maximum amount of kinetic energy
is lost [and appears as some other type of energy]).
This makes the three statements parallel in the sense we are looking at
what has happened to the KE. Also, when stated this way, it is natural
for students to wonder, "Why isn't the minimum allowed final KE zero?"
And this allows you to discuss how conservation of energy is different
from conservation of momentum.
I might also mention that I have also heard the words...
(4) Completely Inelastic: the final KE is zero. And if you talk about
this with students you can discern the initial conditions necessary
before this could occur. So having words (completely inelastic) to
describe the zero final KE situation seems worthwhile.
Even though I am familiar with the definition of Perfectly Inelastic as
the type of collision in which the objects stick together, I don't
prefer that. I prefer the "minimum KE is conserved" definition. It
focuses the discussion on the relevant physics, i.e. conservation of
momentum does not allow any more KE to be lost.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu