I should add that John Clement's point is well taken. Conserving a non zero
VECTOR momentum not only demands moving mass, it demands a predominance of
motion in a specific direction, because the momentum to be conserved is a
vector. Kinetic energy can be conserved via randomized motion, preserved
momentum defines a preserved direction.
Bob Sciamanda
Physics, Edinboro Univ of PA (Em) http://www.winbeam.com/~trebor/
trebor@winbeam.com
----- Original Message -----
From: "Bob Sciamanda" <trebor@WINBEAM.COM>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Saturday, February 19, 2005 3:13 PM
Subject: Re: collision question
| 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
|| > | >
|| > |
|| > |
|| >
||
|
|