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Newton's Third Law

  1. Oct 3, 2004 #1
    If there was a block on a table what would be the normal force between the block and the table. If you could describe the free body diagram that would be great too.
  2. jcsd
  3. Oct 4, 2004 #2


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    Science Advisor

    The normal force between the block and the table is just the weight of the block.

    A "free body diagram" would show the weight vector of the block acting downward on the table and an equal force vector acting upward on the block.
  4. Oct 4, 2004 #3
    normal force

    Assuming that the only other important force acting on the block except normal reaction is the force of gravity, then the normal force depends upon the angle of the table with the horizontal.

    In the most probable case when the angle is 0 deg., the normal reaction is equal to the force of gravity in magnitude (but upwards), not because of Newton's third law but because of Newton's first law. Since, the block is not accelerating, forces on it must be balanced, so normal force has to be equal to the force of gravity.

    In case the table is at an angle, normal recation is equal in magnitude to the component of gravity opposite to it.

  5. Oct 4, 2004 #4
    I just wanted to say an important fact about the free body diagram, that makes students confused:
    Just put the forses that ACTs on the specified body and not more!
    It may seem very simple, but I have seen lots of mistakes about it.
    Good Luck!
  6. Oct 4, 2004 #5

    Doc Al

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    Staff: Mentor

    Assuming the ordinary situation, a block resting on a horizontal table, then a free body diagram of the block would show:
    - its weight acting down (agent = the earth)
    - the normal force acting up (agent = the table)

    Since we know the block is in equilibrium, the magnitude of the normal force must equal the block's weight.

    So far, none of this has anything to do with Newton's Third law. Did you have a question about it?
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