Re: [Phys-l] Interactive Physics Simulations

["Jeffrey Schnick" <JSchnick@Anselm.Edu>]

In Engineering Mechanics, Dynamics, Pearson Prentice Hall 2004, 10th
Edition; R.C. Hibbeler, defines a coefficient of restitution e (in the
reference frame in which all motion is along the line of impact for the
case in which the forces the objects exert on each other also along the
line of impact and there are no external forces acting on either object)
as the difference in the speeds of the colliding objects after the
collision to the difference in the speeds of the colliding objects
before the collision.  When this ratio is zero, Hibbeler calls the
impact plastic or inelastic and says that the objects stick together.
In such a case there is a period of deformation but there is no
subsequent restitution.  When the ratio is 1,  Hibbeler calls the
collision perfectly elastic.  He uses the adjective for the elastic
case.  To me this implies that for 0 > e > 1 the collision is partially
elastic meaning there is some restitution but one or both of the objects
fail to recover their original shape.  Thus, for an elastic collision, e
> 0; and if the collision is perfectly elastic, e=1.  This suggests that
the "totally" in the expression "totally inelastic collision" is
redundant.  I am a fan of the redundancy in this case.

The total momentum of the system is the same before and after the
collision, only in the idealized case of the perfectly elastic collision
can the same be said for the macroscopic kinetic energy of the system.

Jeff Schnick

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Rick
Tarara
Sent: Thursday, October 12, 2006 10:36 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Interactive Physics Simulations

I maybe haven't read this thread carefully enough, but could someone
define what a 'totally' inelastic collision actually is?  In my mind, an
elastic collision conserves both momentum and kinetic energy while an
inelastic collision conserves momentum but not KE.  It's the 'totally'
that is confusing me here.

Rick

----- Original Message -----
From: "John Mallinckrodt" <ajm@csupomona.edu>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Thursday, October 12, 2006 10:21 AM
Subject: Re: [Phys-l] Interactive Physics Simulations


> Hugh wrote:
>
> > Of course, modeling a "real" inelastic collision is not simple, since
> > it necessarily involved distortion of the colliding objects and a
> > certain amount of microscopic intermingling to keep them together
> > and the collision time is relatively longer than with an inelastic
> > collision. But IP treated inelastic colisions in a fairly
> > simple-minded way, and I know that if I didn't do something
> > "unphysical" in constructing the collision, the two objects would
> > gradually drift apart, and this was especially true if one of the
> > objects was rotating.
>
> I think there has been some confusion in this thread engendered by
> conflating "inelastic collision" with "sticking."  No "intermingling"
> is required for an inelastic collision, but it may be for
> "sticking."  Except in the most special of cases--e.g., head-on
> collisions between nonrotating objects--colliding objects WILL move
> away from each other after a totally inelastic collision unless they
> ALSO "stick."
>
> John Mallinckrodt

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