I hadn't recognized this restriction on the question regarding
"dissipation".
When a hot object dissipates energy to its cooler surroundings, is this not
an interaction with the external environment? I think that you want to
restrict the word "dissipation" to only transfers to internal modes - why?
Bob Sciamanda
Physics, Edinboro Univ of PA (Em) http://www.winbeam.com/~trebor/
trebor@winbeam.com
----- Original Message -----
From: "John Barrer" <forcejb@YAHOO.COM>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Saturday, February 19, 2005 10:19 AM
Subject: Re: collision question
| --- 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
| >
|
|