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Action and reaction are equal and opposite,but why dont they cancell ?

  1. Jul 15, 2014 #1
    Action and reaction are equal and opposite,but why dont they cancell ?...

    Unlike other forces, like for example.. it a body is pressed from top and below with a same force.. the body stays there and its said that the forces cancell out.. why not here ?
  2. jcsd
  3. Jul 15, 2014 #2


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    They do cancel. The net contribution to linear momentum (totalled over both objects) from a third law force pair is zero.

    They don't cancel on a single object because only one of the two forces acts on that object.
  4. Jul 15, 2014 #3


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    The action is the force which you apply.

    The reaction is the force which is applied on you.

    So they cancel as a pair, but you must work out the dynamics by considering action-reaction separately ... like when a ball strikes a wall, or two balls collide.
  5. Jul 15, 2014 #4
    The third law says that the force you exert on an object is equal in magnitude and opposite In direction to the force that the object exerts on you. But, if you are doing a force balance on the object , you include only forces acting on the object, and not forces acting on you.

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