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Homework Help: Is Newton's third law true?

  1. Oct 4, 2007 #1

    I was wondering, is Newton's third law true? Let's say I've an electromagnet turned on at a constant intensity for a sufficiently long time for the field to have reached a point some distance away. At that point is another electromagnet, but turned off. Now I turn on the second electromagnet. Immediately it's attracted to the first electromagnet. But before the field created by the second magnet can reach the first electromagnet, I turn it off. So when the magnetic field of the second magnet finally reaches the first electromagnet, it's not a magnetic dipole anymore and thus won't feel a force. The overall case is that there is no balancing parter to the force felt by the second electromagnet. This seems to be a violation of Newton's third law. What is happening here?


  2. jcsd
  3. Oct 4, 2007 #2


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    Newton's 3.law is true for contact forces.

    For long-range acting forces like the electro-magnetic ones, some of a system's momentum (lost from one of the object) may be carried by the electromagnetic field, rather than being instantly swapped over to the other object of the system (i.e, the instant swapping of momentum pieces between a system's objects is the essence of Newton's 3.law).

    This violates Newton's 3.law, but retains momentum conservation for the system as a whole.
  4. Oct 4, 2007 #3


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    Newton's third law is only valid for contact forces; the law assumes instantaneous changes in momentum.

    Edit: Arildno got there first... :redface:
  5. Oct 7, 2007 #4
    So, when the momentum imparted to the second magnet has a counterpart, but instead of being on an object it is being carried away by the propagating field?

    Incidentally, Griffiths writes that it's possible to formulate classical electrodynamics as an action at a distance theory rather than a field theory. How would that work?


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