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Newton's third law

  1. Oct 31, 2015 #1
    The newton's third law states that the force exerted by body A on body B is equal to the force exerted by Body B on Body A . In this case, isn't the object supposed to be stationary because equal forces are acting on both sides?
    Can someone please explain to me . Thanks
  2. jcsd
  3. Oct 31, 2015 #2
    Which object?
  4. Nov 1, 2015 #3


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    Staff: Mentor

    The force exerted on body A causes body A to move.
    The force exerted on body B causes body B to move.
    These are two different forces acting on two different bodies; Newton's third law says that they will be of equal magnitude and opposite direction.

    Search this forum and google for "cart-horse paradox" for more detail.
  5. Nov 1, 2015 #4


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    Staff: Mentor

    If there are no other forces acting on A and B, then the net force on the system of A and B together is indeed zero. It has an important physical consequence: the center of mass of the system either remains at rest, or moves at constant velocity according to Newton's First Law.
  6. Nov 1, 2015 #5


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    Newton's Third Law applies to the interaction between one body and another and not the sum of Forces on a single Body, which says that you need a net unbalanced force for acceleration to take place. That's Newton 1. Each of the forces acting on a body (say the strings pulling it and the table it rests on) will have equal and opposite reaction forces - the mass pushes down on the table with the same force as the table keeping the mass up (N3). The tension in the string is the same in both directions (N3) - whether or not the string is causing the mass to accelerate (N1).
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