Re: [Phys-l] Interactive Physics Simulations

[Hugh Haskell <hhaskell@mindspring.com>]

At 13:02  -0700 10/12/06, John Mallinckrodt wrote:
> 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.")

Which is exactly the way I would treat these situations in the case 
where the students had not yet encountered rotational kinetic energy. 
My initial example was really limited to one dimensional collisions 
where the angular momentum in the c of m frame was zero. I also agree 
that whether one takes rotational k.e. as a component of internal 
energy or something separate is pretty much a matter of taste.

This thread has been very instructive and interesting, but much of it 
has progressed well beyond where the introductory students I have 
been teaching would be ready to go. You get into some of this in an 
AP-course, but even there the level of sophistication is not what has 
been used in this thread.

Hugh
--

************************************************************
Hugh Haskell
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<mailto:hhaskell@mindspring.com>

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When you are arguing with a stupid person, it is a good idea to make sure that
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					Anonymous