OK, I think I get your question, now: Why cannot the bulk momentum of an
object be transferred ("dissipated") to some internal mechanism?
Suppose a car is moving Easterly with a finite linear momentum. If this
bulk momentum is to be reduced to zero and transferred to some sub-assembly,
one would have to end up with a stopped car and an Easterly traveling
sub-assembly. Even if the sub-assemblies are molecular, the conservation
of momentum requires that the CM of the system retains the original
momentum.
Energy can be dissipated into modes (forms) which do not require motion (non
kinetic energy forms). At least in the Newtonian world, non-zero momentum
requires MOVING mass. p = mv is the entire, complete story of linear
momentum.
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 12:39 PM
Subject: Re: collision question
| The original question was framed in the context of a
| collision in which there is no "premeditated" exchange
| with the surroundings during the brief interaction.
| Some of the original CoM KE "escapes" to the
| surroundings as noise, but the time of interaction is
| assumed to be so brief that any heating of the objects
| that occurs during the crash does not have a chance to
| "escape". Looking at the collision in this context, I
| am still lacking a coherent, straightforward
| explanation (for HS students) why there are several
| mechanisms for CoM KE to diminish but these same
| mechanisms (which for the most part involve vibrating
| particles) do not result in a reduction of CoM
| momentum.
|
| In the limit, when a lump of stiff clay collides with
| a lump of soft clay (on a frictionless surface), it is
| easy to see that the dt's are the same for both lumps
| but the dx's are not. But I don't know that this model
| explains the situation of two colliding lab carts.
|
| Still searching. John Barrere University HS,
| Fresno,CA
|
| --- Bob Sciamanda <trebor@WINBEAM.COM> wrote:
|
| > 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
| > | >
| > |
| > |
| >
|