Re: [Phys-l] Interactive Physics Simulations

[John Mallinckrodt <ajm@csupomona.edu>]

On Oct 12, 2006, at 10:47 AM, Hugh Haskell wrote:

> I don't know how others may treat it, but I look at the collision  in
> the center of mass frame, and define a totally inelastic collision to
> be one in which the post-collision kinetic energy is zero.

Consider the case of two balls that strike each other with a nonzero  
impact parameter and stick together.  I would call that a totally  
inelastic collision, but the two balls do end up rotating about their  
common center of mass with a rotational kinetic energy that can be  
quite substantial.

Better yet, consider the (not quite physically realizable) case of a  
particle attached to a massless vertical stick and another particle  
initially moving horizontally that strikes and sticks to the end of  
the stick.  Is this a totally inelastic collision?  Note that, in the  
CM frame, the final kinetic energy is equal to the initial kinetic  
energy.

Now some (myself included) would claim that rotational kinetic energy  
is "internal" energy.  Thus, in the CM frame, the collision  
completely "internalized" the initial bulk translational kinetic  
energy of the two colliding objects and, in that sense, was indeed  
totally inelastic.  (Note also that the separation speed is zero in  
keeping with the coefficient of restitution based definition of  
"totally inelastic.")

John Mallinckrodt

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

and

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