[Phys-L] Re: collision question

[John Barrer <forcejb@YAHOO.COM>]

--- Bob Sciamanda <trebor@WINBEAM.COM> wrote in part:

> 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.  SNIP

But we were (at least impicitly) confining our
discussion to the "no external forces" condition.  By
extension I take this to mean no transfer of energy or
momentum into or out of the system during the
interaction for which KE and momentum consertvation
are being examined. "Heating the object" clearly
violates this constraint.
John Barrere  University HS  Fresno,CA


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