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I Newton's third law is frame independent?

  1. Oct 7, 2016 #1
    how can you say(prove) that newton's third law is frame independent. you will say that as real forces are frame independent , therefore newton's third law is also frame independent, so tell me how real forces are frame independent?
     
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  3. Oct 7, 2016 #2

    A.T.

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    Per definition.
     
  4. Oct 7, 2016 #3
    Ok I agree, but can you give me a reason or explanation which made scientist or anyone to frame it as a definition. how can you say that this definition is correct.
     
  5. Oct 7, 2016 #4

    A.T.

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    Definitions are neither correct nor false.
     
  6. Oct 7, 2016 #5

    Demystifier

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    This can be proved from the Newton second law
    $$F=m\frac{d^2x}{dt^2}.$$
    The transformation from one inertial frame to another is given by the Galilean transformation
    $$x'=x+vt ,$$
    where ##v## is a constant velocity of the frame relative to the other frame. Since this transformation is linear in ##t##, we have
    $$\frac{d^2x'}{dt^2}=\frac{d^2x}{dt^2} .$$
    Therefore, ##d^2x/dt^2## is frame independent. Assuming that mass ##m## is also frame independent, from the Newton second law above it follows that the force ##F## is frame independent. Q.E.D.
     
  7. Oct 7, 2016 #6

    A.T.

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    Real forces are frame independent even across non-inertial frames, so I assumed this is what the OP asks about. As for the main question, Newtons 3rd Law holds only in inertial frames.
     
  8. Oct 7, 2016 #7
    Yes you are right that newtons third law is frame dependent, because fictitious force can't be felt, its a imaginary force, then how it can have a reaction pair , thanks.
     
  9. Oct 7, 2016 #8
    Yoo right , we can also prove it without using galilean transformation,let S and S' be inertial frame of reference, let 'P' be a particle in S', therefore aPS' = aPS - aS'S, since S' is moving uniform to S , aS'S = 0 , therefore aPS' = aPS , as accelaration of particle is same in both frames and mass is same(here) , force will also be same.
     
  10. Oct 7, 2016 #9

    David Lewis

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    For the same reason time moves from past to future.
     
  11. Oct 8, 2016 #10

    jbriggs444

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    Before asking how a fictitious force can have third-law partner, you must ask whether it has a third-law partner.

    Since it does not have a third law partner, the first question does not arise.
     
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