[Phys-L] Re: Momentum

[Richard Tarara <rbtarara@SPRYNET.COM>]

Maybe I can clarify the question.  I think David knows quite well that once
we use momentum conservation here, then the KE must be 1/2 the initial.
But, how is it that the channels into which the KE is converted, which seem
somewhat random and non-specific, always somehow are able to convert
exactly 50% of the KE into other forms?

I'll take a crack at it.  I think you have to look at it the other way
around.  The conservation of momentum dictates that only 50% of the initial
energy is available for KE in the final system.  We have to keep reminding
ourselves that this energy thing is just a book-keeping system.  Since 50%
is NOT available as KE, it will find its way into other forms--deformations
and thermal energy primarily.  Momentum conservation is primary here and
sets the energy parameters.  [That's my feeble attempt.]

Rick

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Professor of Physics
Saint Mary's College
Notre Dame, Indiana
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> [Original Message]
> From: jbellina <jbellina@SAINTMARYS.EDU>
> To: <PHYS-L@LISTS.NAU.EDU>
> Date: 12/5/2005 8:27:53 PM
> Subject: Re: Momentum
>
> The final state has two particles stuck together, so the mass is
> doubled.  By momentum conservation, the velocity must be half.  Since
> the ke is proportional to the mass and to the velocity squared, the
> total ke afterwards is half what it was in the beginning.  That is
> ideal.  You can ask what happens to energy, but that is a different
> question.  As you mentioned, depending on the circumstances the
> energy can be lost in a variety of pathways.  Have I missed something?
>
> joe
>
> Joseph J. Bellina, Jr. Ph.D.
> Professor of Physics
> Saint Mary's College
> Notre Dame, IN 46556
>
> On Dec 5, 2005, at 7:55 PM, David Abineri wrote:
>
> > In the ideal case of two equal masses colliding linearly where one is
> > stationary and one is moving whereby they stick together after the
> > collision (totally inelastic), the conservation of momentum leads
> > to the
> > conclusion that half the kinetic energy is lost to other forms.  Of
> > course, in the real world, one has sound, deflection of materials,
> > friction at least that will convert the energy to other forms.
> >
> > But what does this really say?  What do the "ideal" conditions mean?
> > Why, regardless of the masses is exactly half of the kinetic energy
> > lost.  When we say "ideal" conditions to we mean frictionless, rigid
> > objects in an airless world?
> >
> > Can someone help make some sense out of this? I wish I could make the
> > question more clear so perhaps it is my lack of clarity about the
> > situation that is the cause of the question.  Any help in clarifying
> > this issue would be appreciated.      David Abineri
> >
> > --
> > dabineri@fuse.net
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