Violation of Newtons Third law

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Discussion Overview

The discussion revolves around the potential violation of Newton's third law in the context of electromagnetism and its implications for conservation laws. Participants explore whether Newton's third law holds under various conditions, particularly in relation to electromagnetic forces and special relativity.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants assert that Newton's third law is often claimed to be violated in electromagnetism, but question the validity of this claim.
  • One participant suggests that the apparent violation arises from omitting the momentum of electric and magnetic fields, which, when included, restores adherence to Newton's third law.
  • Another viewpoint posits that Newton's third law can be seen as a special case of more general conservation laws, which may not hold under certain conditions in electromagnetism.
  • Concerns are raised about the implications of these ideas for the existence of reactionless drives, with one participant stating that conservation of momentum and angular momentum prevents such drives.
  • A participant reflects on the relationship between Newton's third law and conservation of momentum, suggesting that violating one implies a violation of the other.
  • Discussion includes the limitations of Newton's third law in the context of special relativity, where the interaction between fields and particles complicates the straightforward application of the law.

Areas of Agreement / Disagreement

Participants express differing views on whether Newton's third law is violated in electromagnetism, with some supporting the idea of violation under certain conditions while others argue for its validity when considering additional factors like field momentum. The discussion remains unresolved regarding the implications of these viewpoints.

Contextual Notes

Participants note that the discussion involves advanced topics such as the conservation of momentum in electromagnetic contexts and the implications of special relativity, which may not be fully accessible to all contributors.

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TheBlackNinja said:
I've heard many times that Newtons third law is violated in electromagnetism, but I never knew why.
I found this link http://www.google.com/url?sa=t&sour...dkMvK2nhw&sig2=jwk3H90wOMVq8cPtcPk7UQ&cad=rja
saying that actually it is not violated.

Is it correct? Is it violated somewhere else?
It is violated, either that or F=ma is violated.

From the link:
link provided by OP said:
If we sum the magnetic forces of two charged particles on each other, we find this apparent violation of Newton’s third law (after some manipula- tion): [equation elided]

The reason for the apparent violation is that the momentum of the electric and magnetic fields has been omitted. When that momentum is included (an advanced topic), Newton’s third law is again obeyed.
I beg to differ. What is true is that when that momentum is included (an advanced topic), the conservation laws are again obeyed.

One way to look at Newton's third law is that it is a special case of the more generic laws of conservation of momentum and angular momentum. The circumstances under which Newton's third law derives from the conservation laws are when forces are central in nature and depend on position only, and when the field that mediates the force does not itself store momentum or angular momentum. Various electromagnetic forces fail on all three accounts, so why would you expect Newton's third law to hold in such a case?
 
Last edited:
If this is true then why don't we have reactionless drives?
 
TurtleMeister said:
If this is true then why don't we have reactionless drives?
Because momentum and angular momentum are conserved quantities.
 
Probably too advanced for me, so I'll just take your word for it. I've always thought of the third law and the conservation of momentum to be simply two different ways of viewing the same thing. Violate one then you violate the other.
 
TurtleMeister said:
Probably too advanced for me, so I'll just take your word for it. I've always thought of the third law and the conservation of momentum to be simply two different ways of viewing the same thing. Violate one then you violate the other.

Newton's third law is true for Newton's gravity - the sun attracts the Earth and the Earth attracts the sun at the same time - even though both are far from each other. How can each know how hard the other is pulling immediately? In Newtonian physics, there is no upper limit to the speed of things, so there's no problem there. But this is obviously doesn't fit in with special relativity - so Newton's third law doesn't hold in special relativity.

In special relativity, the local exchange of momentum is between fields and particles. A particle feels a force due to a field. But there isn't a simple way to say that a field feels a force due to a particle. So to keep the spirit of Newton's third law for fields and particles, we ascribe momentum to both, and say that momentum is conserved.
 
Oh, I see. Thanks for the simple explanation.
 

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