Force Interactions

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2 total messages Started by J.F.McLaughlin Pittsburgh,PA Wed, 29 Sep 1993 23:40

#1818

Author: J.F.McLaughlin Pittsburgh,PA
Date: Wed, 29 Sep 1993 23:40

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In reply to the gentleman who asked for help regarding the forces between
charges, I would like to add my proverbial two cents worth.  Electrical
forces present a particular challenge since they are either attractive or
repulsive, but I think that part of the groundwork which can lead to
understanding this complication is laid in mechanics.  In particular, we
must stress the interactive aspect of Newton's third law- that is, forces
come in equal but opposite pairs.  I take a bit of liberty to stress the
"active" and "reactive" aspects of these laws by leaning against a wall and
describing the pair of forces involved; if I weren't "actively" pushing
against that wall, it wouldn't react.  As I stand on the floor, I point out
that it "reactively" flexes a bit due to my weight.  I think that this
stress on force pairs, the only way forces _can_ be presented, is absolutely
essential, but note to students that it is perhaps a strange way to think
about things because it is necessary for the mathematics to "work out".
<<Joe>>
      +-+-+-+-+-+-+-+-+-+-+-+
   Joseph F. McLaughlin, Physics
   jxm39.psuvm.psu.edu
   Brentwood High School
   Brentwood (Pittsburgh), PA

Re: Force Interactions

#1820

Author: Rick Matthews
Date: Thu, 30 Sep 1993 12:16

34 lines
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J.F.McLaughlin Pittsburgh,PA (JXM39@PSUVM.BITNET) wrote:
: In reply to the gentleman who asked for help regarding the forces between
: charges, I would like to add my proverbial two cents worth.  Electrical
: forces present a particular challenge since they are either attractive or
: repulsive, but I think that part of the groundwork which can lead to
: understanding this complication is laid in mechanics.  In particular, we
: must stress the interactive aspect of Newton's third law- that is, forces
: come in equal but opposite pairs.  I take a bit of liberty to stress the
: "active" and "reactive" aspects of these laws by leaning against a wall and
: describing the pair of forces involved; if I weren't "actively" pushing
: against that wall, it wouldn't react.  As I stand on the floor, I point out
: that it "reactively" flexes a bit due to my weight.  I think that this
: stress on force pairs, the only way forces _can_ be presented, is absolutely
: essential, but note to students that it is perhaps a strange way to think
: about things because it is necessary for the mathematics to "work out".
: <<Joe>>

I prefer to avoid the whole "action-reaction" nomenclature.  If there
are two 1 coulomb charges a meter a part, which force is the action,
and which is the reaction?

If instead we just say the force of A on B is equal in magnitude and
opposite in direction to the force of B on A, we've said all there is
to the third law, with no confusion.  Give a few illustrations, as Joe
suggested, but I would not talk about which is the action and which is
the reaction.

When fist collides with head, calling one force the action and the
other the reaction is based on intent (psychology?), not physics.

--
Rick Matthews                     matthews@wfunet.wfu.edu
Wake Forest University            919-759-5340   (Voice)
Winston-Salem, NC 27109-7507      919-759-6142   (FAX)

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