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Is newton's 3rd law wrong?

  1. Jul 15, 2009 #1
    Okay, I know it may sound stupid but I read somewhere (long ago) that relativity puts Newton's 3rd law in confutable state.
    I don't remember how the article exactly read but I remember that it has some connections with 0 and 9. It was like ( as in my fade memory I remember ) that newton's 3rd law violates (or doesn't connect, or something related) something (sort of mapping I think) between 0 and 9.

    I am totally confused?
    Does relativity really discards the 3rd law, and if so, how? What about that 0 and 9 connection (or mapping or whatever) ?
    What does this all means?
     
  2. jcsd
  3. Jul 15, 2009 #2

    Dale

    Staff: Mentor

    I have never heard of anything like that. It would be very surprising to me to find that base-10 digits have anything to do with any physical laws. Can you link to your source?
     
  4. Jul 15, 2009 #3
    Newton had three laws, but they only say one thing: momentum is conserved.

    You might be thinking of his second law being invalid. It's sometimes written as F = ma. This isn't correct in relativity. However, another form of this same law, F = dp/dt (where p is the momenum) IS valid.
     
  5. Jul 16, 2009 #4
    Yeah its surprizing to me too. As I said I read that long ago, and some days ago it just struck my mind when I was thinking of relativity. I tried to remind myself of something more, but no luck ( though I could remember that 0 and 9 was in concern of solar system and I remember nothing more than that ).

    For source sorry I cannot link you to anything.
    Anyway thanks for your try.:smile:
     
  6. Jul 16, 2009 #5
    Yeah! I knew that!
    I think I'll have to get something more than just fade memory to discuss something. :frown:
    Anyway thanks! :wink:
     
  7. Jul 16, 2009 #6
    In regard to f=ma (as someone supposed was your question), consider that mass is not constant, but increases the higher the velocity. This is a case where a Lorentz factor has to be used. This is such a minute correction at non-relativistic speeds that it doesn't matter, but at relativistic speeds (where velocity is a significant fraction of c) it can be very significant.

    As to the third law, it's true that it does not strictly hold under relativity. Here's a short 3-page paper that discusses it; you might have read between the differential equations, but it lines it out:

    http://physicsanddrums.com/newtons_third_law.pdf [Broken]
     
    Last edited by a moderator: May 4, 2017
  8. Jul 16, 2009 #7

    ghc

    User Avatar

    Newton's third law says: if a body A exerts a force on a body B, the body B exerts a force on the body A and the two forces are opposite in direction and equal in magnetude.
    Relativity laws says that the interaction (forces) between A and B can't "travel" with a speed higher than c and a change in the force that A exerts on B will take some time to reach B. For exemple, A and B are two electrically charged bodies in vacuum separated by a distance L. If we move A a little bit from its initial position, there will a change the force exerted on B, but B will not know that change before a time T=L/c. During that time B still "feels" the initial value of the force exerted by A, and A, in its new position, "feels" a new value of the force exerted by B. During that time, these two forces are generally not equal in magnetude and are not in oppposite directions.
     
  9. Sep 25, 2009 #8
    Yeah it looks like Newton's third law is going down. Nasa tested the asymetrical capacitor in a vacuum and it propelled itself. Newton's third law was more a statement of belief than anything else as there was no known violation of Newton's third law in nature, but then you don't find cell phones much in nature either. There may be another more practical system that if anyone wants a link to it I will find it for them that proposes a gauge theory that ties the conservation of momentum and conservation of energy together.
     
  10. Sep 25, 2009 #9

    russ_watters

    User Avatar

    Staff: Mentor

    Source please.
     
  11. Sep 25, 2009 #10
    As I said in "Force & S.Relativity" , meaning of simultaneity in relativity is relative. It depends on frame & is not general.
    Let design an artificial experiment. Assume that there are just two bodies A & B in universe that exert force to each other.Suppose we have some inertial frames. In on of the inertial frames at instant T we measure the force A exerts on B and B on A. If third law of Newton was true , this two forces must be equal. but these two measurements in another inertial frame that has velocity relative to the first , are not simultaneous. I think it could be showed that if third law waz true , the force between A & B should be constant regard to time for each inertial frame.

    Good luck
     
  12. Sep 25, 2009 #11

    Dale

    Staff: Mentor

    Newton's 3rd law is a statement of the conservation of momentum which arises from the fact that the Lagrangian is symmetric wrt displacements in space (Google Noether's theorem). SR, GR, QM, and the Standard Model all support the conservation of momentum. You are mistaken.
     
  13. Sep 25, 2009 #12
    There is a lot of information on the Web about the assymetrical capacitor propulsion. One place the video of the test that NASA did in 2003 can be seen is at:
    http://www.metacafe.com/watch/90739...or_test_in_vacuum_at_nasa_nsstc_leeif_facili/

    The NASA report about it is at http://gltrs.grc.nasa.gov/reports/2004/CR-2004-213312.pdf [Broken]

    Another proposed method of getting around the 3rd Law has to do with capturing the exhaust of a rocket using a thermal rectification process. It can be found at this link:
    [crackpot link deleted]
     
    Last edited by a moderator: May 4, 2017
  14. Sep 25, 2009 #13

    russ_watters

    User Avatar

    Staff: Mentor

    Here's the last two sentences of the NASA report, which clearly and directly contradict your assertion:
    Let me be perfectly clear: what you are claiming is not only factually wrong, but it is crackpottery. It is against the forum guidelines to post factually wrong information and crackpot ideas. Mistakes are one thing, but now you know.
    That one is seriously just pathetic. It is the classic crackpot problem: they make the problem just complicated enough that they no longer understand what is going on. Don't post it or such ideas again.

    Question answered, thread locked.
     
    Last edited by a moderator: May 4, 2017
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