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[Phys-l] "inelastic"

On 10/12/2006 11:52 AM, John Mallinckrodt wrote:

Richard Bowman wrote:



A totally (or completely) inelastic collision is one in which the
colliding objects stick together after the collision




[A] I disagree. A totally inelastic collision is one in which the
separation speed of the CM's of the colliding objects is zero JUST
after the collision. That doesn't guarantee, however, that it will
remain zero. If the system has angular momentum, the two objects
will separate unless they ALSO stick together.

[B] Having said that, allow me also to note that "collisions" are tricky
beasts and it isn't always obvious when they "start" or when they
"end." In the case of collisions between objects that interact only
via "contact forces" and that DON'T stick together, one can, perhaps,
say that the collision ends at the instant the last contact ends.
After all, as long as two non-sticking objects remain "in contact,"
they will be exerting repulsive forces on each other. A little
thought may convince you that if the surfaces have ANY resilience
whatsoever, then a truly inelastic collision is impossible UNLESS the
two objects stick together.



I'm confused.

Doesn't part [B] trivialize part [A]? It sounds like we're talking
about a distinction without a difference, i.e. without a physically
realizable difference.

==========

Also FWIW the definition of "inelastic" suggested by part [A] is
new to me ... new and surprising. I would have been happy with
Richard Bowman's definition.

Authors are free to define terms however they like, but I suggest
that if you're going to use "totally inelastic" to include the
possibility of not sticking, you'd better explain this explicitly,
or risk being misunderstood.

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.