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Newton's Third Law

  1. Sep 13, 2011 #1
    If I push a box (mass of 20kg) with a force of 140N, is my acceleration 2m/s^2 in the positive direction? I cause it to accelerate in the same direction as I'm pushing in, and that accelerates at 7m/s^2, but it reacts with 140N also, so shouldn't it have some acceleration in my direction to produce that reaction force?
     
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  3. Sep 13, 2011 #2

    Doc Al

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    The acceleration of some object depends on the net force on the object, not on any forces that the object may exert on other objects.
     
  4. Sep 14, 2011 #3
    If I understand you correctly, acceleration x mass of an object does not give the force exerted on others?
     
  5. Sep 14, 2011 #4

    Doc Al

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    Acceleration x mass of an object just gives you the net force on the object.
     
  6. Sep 14, 2011 #5
    I will assume that your mass is 70 kg.

    If you are floating in space and you exert a 140 N force on a 20 kg object, while you are pushing, it will indeed accelerate at 7 m/s/s. And yes, likewise, while exerting the 140 N force, your center of mass will accelerate in the opposite direction by 2 m/s/s, because Newton's 2nd and 3rd laws apply to both you andthe object.
     
  7. Sep 15, 2011 #6
    Of the forces acting on it? Oooh...I find that a bit less intuitive.

    So with weight, 686N is the force ACTING on it, not the force produced by the 70kg mass under gravity?

    Yeah, I misunderstood Newton's laws, I thought the object would've had to accelerate in my direction to exert a force to react against my action
     
  8. Sep 16, 2011 #7

    Doc Al

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    For a 70kg mass near the Earth's surface, 686 N is one of the forces acting on it. It is the gravitational force that the earth exerts on the mass. And it's also the gravitational force that the mass exerts on the earth. (Newton's 3rd law.)

    But when you apply ΣF = ma to an object, ΣF stands for the net force on the object.
     
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