Re: [Phys-L] feeler-dealer, third law, et cetera

[Philip Keller <pkeller@holmdelschools.org>]

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.


On Wed, Dec 11, 2013 at 10:48 PM, Paul Lulai <plulai@stanthony.k12.mn.us>wrote:

> For clarity...
> the example of a block experiencing a force of 20 N left and another force
> of 5N right for a net of 15N does solve the riddle posed.
> However, it is not a N3L violation. The 15N is the sum of the forces.
>  Theere is an object experiencing 20N right and another experiencing 20N
> left. Likewise there is an object experiencing 5N right and another
> experiencing 5N left.
> Lest someone think this violates n3l.
>
> The n3l issues do exists with simultaneity.  I just wanted to ensure the
> above example isnt read by someone as a violation of Newtons third law.
>
> Have a good one.
> Paul.
>
>
> .:. Sent from a touchscreen .:.
>                  Paul Lulai
>
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