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Law of conservation of 'force'.

  1. Sep 30, 2009 #1
    Is this correct?

    What I mean to say here is that suppose a force (x) applies through a mechanism and all the force finally falls on something...or there's just 1 object on which the force falls on (take the magnitude of the force that falls on this object as y)...so can it happen that x != y?
  2. jcsd
  3. Sep 30, 2009 #2
    Aren't you stating the law of action and reaction?
  4. Sep 30, 2009 #3


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    Staff: Mentor

    Correct: forces come in equal and opposite pairs.
  5. Sep 30, 2009 #4
    The Newton equations read:

    M1a1 = -∂U(X1-X2)/∂X1 = F12

    M2a2 = -∂U(X1-X2)/∂X2 = -F12

    Note that F12 and -F12 enter different equations (whatever masses are). So there should be a relative motion due to interaction.

    The forces come together in the center of inertia R equation:

    Mtot∂R/∂t = F12 - F12 = 0

    That is why they say that internal interactions do not influence the CI motion.
    Last edited: Sep 30, 2009
  6. Sep 30, 2009 #5


    Staff: Mentor

    Force is the change in momentum wrt time, so since the momentum of an isolated system is conserved the change in momentum wrt time is zero which is constant wrt time and therefore also conserved. So you could indeed say that force is conserved for an isolated system.

    However, since the definition of an isolated system is a system with no external force then all you are saying is that if the external force on a system is always zero then the external force on the system is constant. I don't think this represents a new Noether charge or current.
  7. Sep 30, 2009 #6
    If there is a force independent of time and space it is called a constant force, not a conserved.

    The force is something external acting on a particle (see Newton equations). There may be laws of conservation of particle "properties" or ensemble of particle "properties", not of the external things.
  8. Sep 30, 2009 #7


    Staff: Mentor

    The statement "X is conserved" means dX/dt = 0. So if F = 0 (isolated system) then dF/dt = 0 and it is reasonable to say "force is conserved", although I agree with you that it sounds weird and the usual statement would be that it is constant.
  9. Sep 30, 2009 #8
    Hello de logics-
    I can design a conservative system such as a lever, where F1x1 = F2x2, but F1 <> F2 unless x1 = x2 . Is this what you mean?
    Bob S
    Last edited: Sep 30, 2009
  10. Oct 2, 2009 #9
    Where did my post go??...I posed here 'thanks'.

    Actually I think I missed a few posts.
  11. Oct 2, 2009 #10
    This if such a component existed, the law of conservation of momentum would have been trash.


    No, that was not the question actually; an isolated system is out of the question here.

    It appears we have a sort of chaos here && I don't know the meaning of the various variables used.

    What I think, this is a necessity cause if the force applied through this mechanism and to a body is more or less than the actual force applied, the momentum delivered to the body will be more or less than the momentum possessed by the colliding body.

    So there should be some "law of conservation of forces"...it's derived from the law of conservation of momentum.

    Even during lever action, this law holds true.

    Search "law of conservation of forces"...we have only 7 results (yes, I know Google search is VERY inaccurate...like Google desktop and so this post is not shown...maybe it need time to reindex.).
  12. Oct 2, 2009 #11
    Ok then...final question; does this law exit?
  13. Oct 3, 2009 #12

    Doc Al

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    Staff: Mentor

    The "force law" leading to conservation of momentum is Newton's 3rd law. Is that all you mean?
  14. Oct 3, 2009 #13
    Yes, sort of everything is indirectly related...I think.

    Since impulse is a function of force and under any collision, the impulse delivered to another body (B) is equal to the momentum possessed by the colliding body...this is the law; so, by this law there cant be a mechanism which reduces or increases this force without any change in time or area.

    I did a few calculations...and according to them, if such a mechanism does exits, it will be against the law of conservation of energy.
  15. Oct 4, 2009 #14


    Staff: Mentor

    I don't think you understand the standard conservation laws. They all apply only to isolated systems. Momentum and energy are not conserved for non-isolated systems.
  16. Oct 4, 2009 #15
    aaaa...I was wondering there was an application of force from the outside here...if it's inside the system, then it's isolated (and I was not assuming that).

    Anyway, I get what you mean, I'm on the same foot as all of you.
  17. Oct 5, 2009 #16
    So finally am I right?
  18. Oct 5, 2009 #17


    Staff: Mentor

    What is your current understanding?
  19. Oct 5, 2009 #18
    This -

  20. Oct 6, 2009 #19


    Staff: Mentor

    I think that is correct, although rather confusingly worded.
  21. Oct 6, 2009 #20
    :D...ok then thanks.
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