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Is Newton's 3rd law incorrect?

  1. Sep 22, 2015 #1
    is the concept hypthetical
  2. jcsd
  3. Sep 22, 2015 #2
    No it's not incorrect...It is applicable for macroscopic bodies but not applicable for microscopic bodies.
  4. Sep 22, 2015 #3


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    Microscopic as well.
  5. Sep 23, 2015 #4
    To every actions there's equal and opposite reaction is not hypothetical.
    It is true for microscopic and macroscopic bodies as the sizes of forces acting on bodies are acting equally.
    But we can't say that the reaction is instantaneous as when a chemical reaction takes place the reaction doesn't take place in seconds it does take time.
  6. Sep 23, 2015 #5


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    Chemical reactions are not what is being referred to in Newton's third law.
  7. Sep 23, 2015 #6


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    No. But why are you thinking it might be?
  8. Sep 25, 2015 #7
    Newton's third law work for every object microscopic as well as macroscopic. And it is instantaneous just think about the fact that when someone shoots a bullet from the rifle or any gun the recoils occur at that moment only and it occurs in the opposite direction of bullet fired
  9. Sep 25, 2015 #8
    It can't be instantaneous in the sense that c is the limiting speed of interactions.

  10. Sep 25, 2015 #9
    Albert Einstein proved through mathematical equations that the carriers which carry the information of gravity calles gravitons work at the speed of light so gravity works as fast as light
  11. Sep 25, 2015 #10


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    So what is your point? What does the transfer of the reaction from one part of the gun to another or to your shoulder have to do with Newton's third law?
  12. Sep 25, 2015 #11
    Both forces can still be the same and in opposite directions at the same time, because both forces will be delayed by the same amount.
  13. Sep 25, 2015 #12


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    Consider two equal positive charges q1 and q2. q1 is traveling along the x-axis in the +x direction, and and q2 is at rest on the +y-axis, a distance y away from the origin. Calculate the Lorentz forces ##\vec F_1 = q_1 (\vec E_2 + \vec v_1 \times \vec B_2)## and ##\vec F_2 = q_2 (\vec E_1 + \vec v_2 \times \vec B_1)## at the moment that q1 passes through the origin. Are those two forces equal in magnitude and opposite in direction?
  14. Oct 6, 2015 #13
    if i give a 10 newton force to an object say a steel scrap then the force will transfer automatically to sound energy,heat energy etcetc and during this time there will be a loss in energy which in return may give 9.5 or 9.8 or so but it cant give exact or equal to 10 newton.....
  15. Oct 6, 2015 #14
    It can if the interaction is locally only. Of course this requires a much more general concept of "body".
  16. Oct 6, 2015 #15
    Can you expand a bit?

  17. Oct 6, 2015 #16
    This description is flawed. You don't "give" a force and then receive another one back.
    The force is a measure of interaction between two objects. If there is an interaction between you and another object, you and the object simultaneously act with a force of 10N on each other. If there are other forces acting at the same time, they are part of other 3rd law pairs. They can decrease the energy of the system, maybe.
  18. Oct 6, 2015 #17
    The interaction between two distant charged particles can't be instantaneous but the local interaction between the particles and their electromagnetic field is instantaneous. Newton's third law applies to this situation if photons or even virtual photons are considered to be "bodies".
  19. Oct 6, 2015 #18
    This is not related to Newton's 3rd law. Newton's third law says that the force you exert on the steel scrap is the same as the force that the steel scrap exerts back on you.
  20. Oct 6, 2015 #19


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    To amplify even a bit more: you don't "give" a force and then receive another one back because that implies that they are separate events. They aren't. The application of/exchange of forces is a single event.
  21. Oct 7, 2015 #20
    It seems very very "stretched" to me, considered that many physicists consider virtual photons as nothing else than a mathematical "trick".

  22. Oct 7, 2015 #21
    I don't think that graviton is a proved particle yet.is it?
  23. Oct 7, 2015 #22


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    Next time, when you make a thread asking for something like this, please also include THIS as your "supporting argument" in that very first post. Otherwise, your question just hangs there without any kind of justification or impetus on why you would ask such a thing (did it come to you in a dream?).

    Furthermore, as nasu has replied, we can also correct the mistake in your understanding on why you would think of such a thing based on this example. It is a more CONCRETE and clearer way to address such a mistake, rather than some hand-waving description on why your understanding is faulty.

    As a reminder, please note that the PF Rules require that you present a clear and as complete of a post as possible when presenting a question such as this.

  24. Oct 7, 2015 #23
    Isn't the third law a mathematical "trick" too? There is no reason why forces must cancel each other pairwise out. Conservation of momentum just requires that the sum of all forces is zero in inertial systems. For example, if there are three bodies A, B and C this would allow that

    A exerts a force FAB to B but no force to C
    B exerts a force FBC to C but no force to A
    C exerts a force FCA to A but no force to B


    FAB + FBC + FCA = 0

    The third law does not allow such ternary interactions and requires to describe the same situation with three binary interactions. Due to the superposition principle this is always possible but it is impossible to distinguish the two cases experimentally. We are using the binary interactions because they appear to be useful. But we cannot show that they really exist in systems with more than two interacting bodies. In this sense they are just a mathematical "trick". But there is nothing wrong with it.
  25. Oct 7, 2015 #24
    other forces means?specfy
  26. Oct 7, 2015 #25
    I don't know, those ones you had in mind when you described dissipation of mechanical energy.
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