Can Newton's Third Law Of Motion Be Violated?by alexanderleg Tags: electrodynamics, magnetism, newton, newton's third law 

#1
Oct1011, 10:29 AM

P: 1

I have read in several books and also in this site that the Newton's Third Law does not hold true for "Action at a distance". Specifically, in the case of Electrodynamics.
Can someone explain to me, what actually happens in the case of electromagnetism? I've read that the actionreaction forces are not collinear that is 'weak form of third law'. What exactly does this mean? Early this year, due to lapse of my brain functioning, I committed a serious mistake of neglecting the third law for my Google Science Fair project. Take a look: https://sites.google.com/site/levitationp2wreduction/ You'll understand how flawed the idea is (and maybe have some pity on my foolishness?) Though, deep inside, I still hope that the third law gets violated so that my idea works. 



#2
Oct1011, 03:31 PM

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Newton's third law does hold true for action at a distance in Newtonian mechanics, as in the Newtonian gravitational field. If the sun exerts a force on you now, you exert the same force on it now (not 8 minutes later). In Newtonian mechanics, conservation of momentum is derived from the third law.
However, in special relativity, we expect there to be an upper speed limit to the interactions between particles. Thus the force between two charges at a distance is described as being mediated by an electromagnetic field, with changes propagating at the speed of light. We'd like there still to be "Newton's third law" locally, like "the charge exerts a force in the field", and "the field exerts a force on the charge". However, it is difficult to say what "the charge exerts a force in the field" means. Instead, it is easier to assign momentum to the field, and to say that momentum is conserved. Thus in special relativity, one takes conservation of momentum as primary, not derived, preserving the spirit, but not the letter of the third law. 



#3
Oct1011, 04:31 PM

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#4
Oct1111, 09:38 AM

P: 89

Can Newton's Third Law Of Motion Be Violated?
I was fairly sure that Newton's Third Law always held true, never hearing of it not holding true. In electromagnetic things, it's fairly simple. Think of it this way:
Particle A has a charge of +1C. Particle B has a charge of +2C. The force is given by [tex]F = \frac{kQ_{1}Q_{2}}{r^{2}}[/tex] This force acts identically upon both particles, hence Newton's Third Law holds. 



#5
Oct1111, 10:06 AM

Mentor
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Newton's Third Law can fail in a number of cases:
In more advanced physics, it is the conservation laws that reign supreme. Newton's third law derives from the conservation laws with the assumption that forces act in pairs, act instantaneously, and act along the line connecting particle pairs. Drop those assumptions and you have to drop Newton's third law. You do not have to drop the conservation laws, however. In even higher level physics, the conservation laws themselves can be derived from the very nature of space and time. 



#6
Oct1111, 10:10 AM

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