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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 exerts 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 collection of forces.


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