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Action Reaction

  1. Aug 22, 2015 #1
    (a) Newtons III law : Every action has equal and opposite reaction

    By (a), if I apply a force of 10N on an object, it sholudnt move, as it has equal reaction, by friction of -10N so as to stop it.

    Isnt that logic correct?
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  3. Aug 22, 2015 #2


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    You should find a more precise formulation. How many bodies are involved in Newtons III Law, and how many in your scenario?
  4. Aug 22, 2015 #3


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    The equal and opposite reaction is due to the second body acting on the first.

    Another statement of Newton's third law for two objects A and B would be...
    "the force on A due to B is equal and opposite to the force on B due to A"

    If I push on a block of ice on a smooth floor, the ice will accelerate.

    The forces on that block of ice, are:
    -a large applied force due to my pushing,
    -a smaller friction force in the opposite direction,
    -gravity pulling down,
    -and the smooth surface pushing up on the object, so that it does not sink through the floor.

    However, as I apply a 10 N force on the block, the block is applying a 10 N force on me, pushing back on me exactly as much as I on it.

    There might seem to be a sort of paradox here, but the important thing to consider is that it's only the forces acting on an object that will determine its acceleration. The forces it imparts on other objects have no direct effect.
  5. Aug 22, 2015 #4


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    That is a very poor formulation of Newton's third law, which states that "When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body .

    It is totally wrong. The friction is not reaction to your push. What happens if you push something on ice? will not it move?
    Friction is force of interaction between bodies in contact. That force opposes relative motion.
  6. Aug 22, 2015 #5
    So what I am missed in my logic?
  7. Aug 22, 2015 #6


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    You stated that you are pushing on an object with a 10N force.

    As everyone here has tried to point out, the equal and opposite reaction is the 10N force of the object on you. But that is a force on you, not a force on the object. The object is still subject to the 10N force and can still move as a result of that force.
  8. Aug 22, 2015 #7


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    ...or not, if there is also friction. The point you are missing, Ajit, is that the friction interaction is a second force pair. This is a very common misconception.
  9. Aug 22, 2015 #8


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    There is a massive difference between conducting the experiment in deep space, where you can say there is only the 10N you apply and when the object is on a sticky table, where the 10N of friction force also is introduced.
    The Third Law applies in both cases, though. The hand providing the 10N force will 'feel' a 10N force pushing back at it in both cases. That is the 'reaction force' that people talk about. Of course, the hand, out in space will be accelerating the object so you will have to chase after it in order to maintain your 10N force on it (and its reaction).
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