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Newton's third of motion

  1. Oct 28, 2011 #1
    1. The problem statement, all variables and given/known data
    The law states that "Whenever one body exerts a force on another, the second exerts an equal and opposite force on first." But then, why is there acceleration?

    2. Relevant equations

    3. The attempt at a solution
    If there will be force responding to the first one with equal magnitude, then there won't be any net force.Then why is there acceleration?
  2. jcsd
  3. Oct 28, 2011 #2
    One must keep in mind that the two forces mentioned in Newton's 3rd law act on two different bodies.
  4. Oct 28, 2011 #3
    but, for example. When I push a wall, I will exert a force on the wall, and the wall will exert a force on me.
    In another case, when I am pushing a person with 100N, and that person also pushes me with 100N, in opposite direction, then none of us will move. How do you explain that?
  5. Oct 28, 2011 #4
    Consider a person being pushed by me by 100N.
    To find whether he will accelerate or not one has to see if there are other forces acting thus contributing to increase or decrease the effect of the !00N that I am applying.
    That is to find HIS acceleration one is interested in the forces acting ON HIM and not on me.
  6. Oct 28, 2011 #5
    Oh, so what you are saying is that it's other forces which make the person accelerate, right?
  7. Oct 28, 2011 #6
    What I said was that to find the acceleration on a certain mass, one has to find the forces acting on THAT mass. That is what Newton's 2nd law says.
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