[Phys-L] Re: Momentum

[Bernard Cleyet <anngeorg@PACBELL.NET>]

This reminds me of the more easily answered question.  How does the wall
know just how much to push back when one pushes against it?

"... it will find its way into other forms--deformations
> and thermal energy primarily."

BUT how does it know when to stop the conversion? *


My impression is that is the question. I suppose we could wait for
clarification, but then we wouldn't be able to reveal our personalities.
  J.D. was predictable; R.T. and J.B. I don't remember well enuff to
have predicted.

I missing something too, as I can't think of a microscopic answer that
fits all contingencies, as the one that does for the question in the
first paragraph.

The examples I immediately think of are:  a putty pall and anything
else,  magnets,  velcro, capture w/ e.g. a trap door.  In each case the
micro. description of dissipation is difficult and indeterminate.  How
does the putty know to warm up so its specific heat cap. times temp rise
is equal to 1/2 the initial KE?

bc, puzzled.


p.s. J.D. answered the ideal question.


* Eureka! but I'll wait.  [as I was about to push the button.]


Richard Tarara wrote:
> 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
>
> ***************************************************
> R.W.Tarara
> Professor of Physics
> Saint Mary's College
> Notre Dame, Indiana
> Free Physics Instructional Software
> Windows & Mac
> www.saintmarys.edu/~rtarara/software.html
> ***********************************************
>
>
>
>
> > [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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