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[Phys-L] Re: A Third law question



This is a "straw dog" argument. Momentum conservation comes quite
naturally from applying Newton's Laws to collisions. N3 gives the
forces as equal and opposite, the collision times are the same,
N2 gives the momentum changes of the colliding objects to be
equal and opposite. Momentum conservation becomes a simple
consequence of Newton's Laws - not an ad hoc quantity invented
for the purpose of rationalizing data. Starting from momentum and
then deriving Newton's Laws is certainly equivalent, but I don't
see how it is superior. Both approaches require careful study of
various simple physical interactions, each can then be used to
produce the other, and they both have the same problem with the
definition of mass.

If a text or instructor is not making that connection then there
is a serious lack of physical understanding on the part of the
author or instructor.

Bob at PC

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-
L@list1.ucc.nau.edu] On Behalf Of Hugh Haskell
Sent: Wednesday, October 12, 2005 9:42 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: A Third law question


I would agree that, in a traditional setting, force and
momentum
would likely be only weekly coupled, since they are typically
taught
as essentially unrelated topics, so it isn't likely that most
students would see the connection in a first course.