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Evil torque problem

  1. Jan 15, 2005 #1
    Hi

    I have a problem for which the given solution makes no sense to me.

    A 200 kg plank of length 2m is attached by a hinge at one of its ends to a wall. It is held in place horizontally by a string. The string breaks and I am supposed to find the torque at that point. (See attached image).

    I have that torque = r x F which gives me a torque vector with a magnitude of 9.8 m/s^2 * 2m * 200kg = 3920 Nm.
    The solution the book gives, however, is half of that, ie 1960Nm.

    Could somebody either reassure me that the book is wrong or clarify where I made a mistake?

    Thank you,
    tonix
     

    Attached Files:

  2. jcsd
  3. Jan 15, 2005 #2

    dextercioby

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    Can u see why your answer is twice more??Is it F,or is it "r"??

    Daniel.
     
  4. Jan 15, 2005 #3
    Well, I don't know why it is twice more. That is the problem. It seems that r should be 2m for sure. I suppose it could be F since the gravity is only acting on half the mass?
     
  5. Jan 15, 2005 #4

    dextercioby

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    Nope,gravity affects all mass...Think about the point of application of the gravity force and its whereabouts wrt the rotation point of the rod...

    Daniel.
     
  6. Jan 15, 2005 #5
    ahh... so the overall point of application of the gravity force is in the middle of the rod and therefore r is only 1m instead of 2m?
     
  7. Jan 15, 2005 #6

    dextercioby

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    It's actually in the center of mass,but,assuming the mass is distributed uniformly throughout the rod,it would be exactly at the middle.
    Yes,that distance is only of 1 meter.


    Daniel.
     
  8. Jan 15, 2005 #7
    That's what I meant to say. Thanks!
     
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