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Violation of Newton's third law

  1. Jul 30, 2015 #1
    when a stone is dropped in a beaker containing water,it sinks.But according to Newton's third law of motion the water should also exert an equal force upwards.......so the stone should float. This violating the third law but why?
     
    Last edited by a moderator: Jul 30, 2015
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  3. Jul 30, 2015 #2
    Acceleration is based on the net force on each object.

    Newton's third law says that the force of one object on another is equal and opposite.

    The gravitational force between the stone and the earth is making the stone sink.

    The water is not exerting an upward force on the stone equal to the force of gravity between the stone and the earth.
     
  4. Jul 30, 2015 #3

    A.T.

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    Equal to what?

    If you think it does, you should explain why.
     
  5. Jul 30, 2015 #4

    DTM

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    The force of the stone on the water, is equal and opposite the force of the water on the stone. Call these forces magnitude Fw
    The force of gravity on the stone, call this Ws is greater than the force of the water on the stone Ws>Fw , Thus the stone continues to accelerate downward through the water.
    The force of gravity on the Earth is equal and opposite the force of gravity on the stone. So Ws is pulling down on the stone, and Ws is pulling up on the earth.
     
  6. Jul 30, 2015 #5

    Drakkith

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    The force of gravity from the Earth on the stone is equal to and opposite of the force of gravity from the stone on the Earth, in accordance with the third law.

    The force exerted on the stone by the water is a different force with a different force pair. See the above responses.
     
  7. Jul 30, 2015 #6
    To clarify what the others have said, the force exerted by the water on the stone is a viscous drag force. This same drag force is also exerted by the stone on the water causing the water to be displaced as the stone accelerates due to the gravitational force.

    Additionally, the gravitational force exerted by the earth on the stone is equal to the gravitational force exerted by the stone on the earth. Therefore, Newton's 3rd law in all cases is not violated.
     
  8. Jul 30, 2015 #7
    You could have the same situation in air if that helps you intuitively. Air does exert a force just not very big.
     
  9. Jul 30, 2015 #8
    Actually in all cases the force varies with velocity and can grow exponentially large enough to balance the gravitational force when the terminal velocity is reached.

    And of course there is also a buoyant force acting on the stone.
     
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