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I Resultant force after adding a force to a couple moment?

  1. Oct 28, 2016 #1
    I don't get it. 24mumo6.png

    Imagine there is a stick leaning aganist a wall.
    There is a moment couple somewhere on the stick.
    The end of the stick hits the wall and the wall acts against it.

    The sum of moment is 0 but since the moment couple forces cancel each other, the resultant force is the wall reaction force! which means the stick will move away from the wall!

    But in reality i think the whole structure is stationary but not this!!!!!
     
  2. jcsd
  3. Oct 28, 2016 #2

    BvU

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    Any difference with this picture ?
    upload_2016-10-28_16-52-35.png
     
  4. Oct 28, 2016 #3

    A.T.

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    Its center of mass will accelerate.

    What makes you think this?
     
  5. Oct 28, 2016 #4

    David Lewis

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    If the stick moves away from the wall, the reaction force goes to zero. (At the point where the stick contacts the wall, the stick compresses the wall, and the wall compresses the end of the stick.)
     
  6. Oct 29, 2016 #5

    A.T.

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    At some time point it will lose contact, but it stays in contact for a while and the contact force accelerates it.
     
  7. Oct 30, 2016 #6

    David Lewis

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    There is no net resultant linear force associated with couple itself. Linear forces cancel for a couple, so the applied load is a pure moment. The reaction force supplied by the wall creates a moment of force that is equal and opposite to the couple.
     
    Last edited: Oct 30, 2016
  8. Oct 30, 2016 #7

    A.T.

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    Of course there is. It's equal to the contact force from the wall.

    There is no reason for these moments to be equal and opposite.
     
    Last edited: Oct 30, 2016
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