[Phys-L] Re: Momentum

[jbellina <jbellina@SAINTMARYS.EDU>]

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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