[Phys-L] Re: collision question

[Bob Sciamanda <trebor@WINBEAM.COM>]

I'm not sure what is the underlying principle being invoked here (that a
vector quantity must be conserved, or cannot be dissipated?), but consider
the case of a ferromagnetic object.  The molecular magnetic moment vectors
(m) may add up to a non-zero overall Moment Vector (M) for the object.
But, simply by heating the object I can not only dissipate this M - I can
reduce it to zero, without a compensating M arising elsewhere.  No
conservation, easy dissipation, of the vector quantity M.

-Bob Sciamanda


On Thu, 17 Feb 2005 15:45:15 -0600, John M Clement <clement@HAL-PC.ORG>
wrote:

> While one can appeal to all kinds of arguments about how momentum is
> transferred by impulse, often these arguments may not be convincing to
> students.
>
> One vital difference between momentum and energy is that momentum is a
> vector, while energy is a scalar.  When momentum is transferred the
> direction prevents it from apparently disappearing.  Even if it is
> transferred to one atom, it should still be there when you add up all
> the contributions.  OTOH energy being a scalar can be transferred to
> internal locations such as molecules as either potential or kinetic
> energy.  So it apparently disappears.
>
> This propery of energy makes it more confusing than momentum to the
> students, hence the increased scores on evaluations observed by Laws,
> Thornton, & Sokoloff when momentum is taught before energy.  Of course
> they also has stripped two dimensional cases from their early
> curriculum and only do them after they have exhausted one dimensional
> physics.
>
> John M. Clement
> Houston, TX
>
> > A related question: Does anyone have a
> > conceptual-level explanation for why there are no
> > momentum-dissipative mechanisms in collisions? On the
> > micro level, if some of the original KE goes to
> > increasing the internal vibrations (and internal KE)
> > of the objects, why does momentum not get transferred
> > in the same fashion? I've never heard (or been able to
> > give) a satisfactory explanation to this question.
> > Thanks for any pearls of wisdom.
> > John Barrere  University HS, Fresno, CA