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Re: [Phys-l] "inelastic"



John Denker wrote:

... John Mallinckrodt wrote:

[A] ... 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.

Possibly, yes. However, I would still maintain that the concept of "inelasticity" is separate from that of "stickiness." One can approach perfect inelasticity arbitrarily closely without stickiness.

Also FWIW the definition of "inelastic" suggested by part [A] is
new to me ... new and surprising.

I thought it was pretty standard. As others have written, perfect inelasticity is generally considered to mean zero coefficient of restitution which is the same thing as saying zero separation speed. Stickiness may or may not be required to accomplish that, but I've never heard it specified as part of the definition of perfect inelasticity.

John Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://outlawsofphysics.com>