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I Exceptions to the Laws of Physics: Newton's Third Law

  1. Sep 5, 2016 #1
    I've been noticing that there are exceptions to every scientific law. For Example, with Newton's First and Second Laws are totally invalid at the quantum level, but I'm failing to find such an example for Newton's Third Law. Is anyone able to help me?
  2. jcsd
  3. Sep 5, 2016 #2
    There are no known exceptions to Newton's first and second law. Why not? Because every law also has a domain of applicability. So Newton's law are not incorrect, rather they are not applicable at quantum level.
  4. Sep 6, 2016 #3
    I think what both of you are trying to say is the same thing. Akshat does not say that Newton's laws are incorrect but it can not be applied at the quantum level. Am I making some mistake?
  5. Sep 6, 2016 #4
    Correct. I'm saying that according to the Law of Exceptions, no law of physics is truly universal. The first two laws' exceptions are in the quantum world, but I'm struggling to find the exception for the third law.
  6. Sep 6, 2016 #5


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    Whoa! Go back a bit. Where exactly in QM are Newton's 1st and 2nd laws "totally invalid"? What do you think you get when you apply Ehrenfest theorem in QM?

  7. Sep 6, 2016 #6
    I used the word invalid because Laws 1 and 2 involve force, and the classical concept of force doesn't exist. And I can't say anything about Ehrenfest, because I don't know anything about it. I'm just an incoming first year student.
  8. Sep 6, 2016 #7


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    What do you mean that the concept of force doesn't exist in QM?

    In a Schrodinger equation, when you write the central potential for a hydrogen atom, do you think that the gradient of that potential isn't a "force"?

    I am trying very hard to figure out WHERE you got this idea that (i) there is no such thing as a "force", and that (ii) Newton's 1st and 2nd law are invalid in QM. Your explanation that you have given in this thread has been very vague. How about you either cite me your source, or give me a specific example to support your claim.

    But yet you seem to not be shy about making these claims in this thread. And what's to prevent you from looking it up?

    http://www.reed.edu/physics/faculty/wheeler/documents/Quantum[/PLAIN] [Broken] Mechanics/Miscellaneous Essays/Ehrenfest's Theorem.pdf

    Look at the form that is practically equivalent to Newton's 2nd law!

    Last edited by a moderator: May 8, 2017
  9. Sep 6, 2016 #8


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    The "law of exceptions" is not a recognized law of physics. Posts about it will be deleted.

    Stick to the discussion about Newton's laws
  10. Sep 6, 2016 #9
    Take two protons moving with velocities at right angles to each other. The magnetic forces exerted by each on the other do not follow Newton's third law.
  11. Sep 6, 2016 #10


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    How so?
  12. Sep 6, 2016 #11
    Proton 1 (p1) moving in the positive x direction, currently at some position on the positive x-axis. Proton 2 (p2) moving in the positive y direction, currently at some position on the positive y-axis.
    The magnetic field of p1 at the position of p2 is in the positive z direction (Biot-Savart law). So the force on p2 is in the + x direction (Lorentz force).
    The magnetic field of p2 at the position of p1 is in the - z direction. The force on p1 is in + y direction.
    So the two forces are not opposite to each other.
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