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-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Richard Tarara
Sent: Thursday, December 12, 2013 11:15 AM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] feeler-dealer, third law, et cetera
I'm not sure I'm comfortable with the electron/proton example as a violation
of the third law. Unlike situations like that presented below by Philip, even
at the intro level I would never say that electron A exerts a force on proton B.
Rather I would state that moving electron A produces a magnetic field C that
interacts with proton B producing a force on that proton. At the same time,
moving proton B produces a magnetic field D that interacts with electron A.
Even at the most elementary level, we HAVE to talk about the fields with the
moving charges, but don't need to bring in fields to a hand/ball interaction. It
is the field/charge interaction that will deflect the
particles. I'm certainly not absolutely convinced there is a third-law
violation here, but if so, I suspect it is because we have left the "Newtonian
world".
rwt
On 12/12/2013 7:23 AM, Philip Keller wrote:
When I state the third law in class, I begin: "when an object A exertscollection of forces.
a force on object B..."
This formulation evades the issue of whether the third law applies to
the sum of the forces. And I am glad to evade that question: I care
about the sum of the forces that act ON object A because that sum
determines object A's acceleration. I don't care about the sum of the
forces exerted BY object A because it is likely that they are acting
on an assortment of objects, each which are experiencing their own
I have often told my students that I am unaware of any exceptions to
this law and that if there were any exceptions, they would violate the
law of conservation of momentum. But now I see from Bruce's post
about the electron adn proton pair that I have some reading to do.
--
Richard Tarara
Professor of Physics
Saint Mary's College
free Physics educational software at
www.saintmarys.edu/~rtarara/software.html
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