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# [Phys-L] Re: conservation of angular momentum question

The two forces of a couple act on the same system, but not necessarily
on the same object. Although the two forces in an interaction pair act
on different objects, they can both act on the same system, if they are
internal forces. Thus, one can legitimately discuss the couple
associated with an internal interaction pair; however, the couple will
be zero in most cases, and, therefore, of no interest.

Daniel Crowe
Oklahoma School of Science and Mathematics
Ardmore Regional Center
dcrowe@sotc.org

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-L@list1.ucc.nau.edu] On
Behalf Of Frohne, Vickie
Sent: Tuesday, March 15, 2005 10:13 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: [PHYS-L] conservation of angular momentum question

This is a great introduction to couples! I'd like to add that in an
action-reaction pair, the two equal and opposite forces act on
_different _ objects. This is a very subtle, but important, point that
is often missed in explanations of applications of Newton's Third Law.
In a couple, the two forces act on the _same_ object. Therefore the two
forces acting as a couple do not comprise an action-reaction pair.

Vickie Frohne
________________________________

From: Forum for Physics Educators on behalf of Daniel Crowe
Sent: Mon 3/14/2005 10:57 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: conservation of angular momentum question

<snip>
Most interesting couples are not associated with interaction pairs
(traditionally, action-reaction pairs), because most interaction pairs
share the same line of action.
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