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Rotational equilibrium

  1. Jan 12, 2008 #1
    [SOLVED] Rotational equilibrium

    1. The problem statement, all variables and given/known data

    This isn't actually a problem, but a portion of our latest lab assignment.
    There is a pole with 3 hooks in it, one in the center at the top and two at varying distances on the bottom. The pole is hanging from a ringstand by the top hook. the two hooks on the bottom are distances of 20cm(left) and 80cm(right). We were required to add weights to each side until the pole reached a point of rotational equilibrium. once equilibrium was reached the left side had 131.43g of weights and the right side had 36.32g of weights, with this information we are asked to determine the mass of the hanging pole. so far i haven't found out how to apply the given information, if anyone has any ideas help would be much appreciated.
     
  2. jcsd
  3. Jan 12, 2008 #2

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    Since this is a homework forum, you are required to show some attempt at solution from your side, however bad you may feel it to be. What is the condn for rot eqlbrm?
     
  4. Jan 14, 2008 #3
    , you know, if you don't like my question feel free not to answer it, but if you have anything productive to add feel free to post, and if you knew what rotational equilibrium meant then you would understand that when the weights cause the hanging pole to balance and no angular motion is experienced by the hanging pole then it is in a state of rotational equilibrium.
     
  5. Jan 14, 2008 #4

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    And you are free to post it anywhere except the HW forums, regardless of your suspicion about my knowledge about rotnl equilibrium. Best wishes.

    To other helpers: Pl don't reply to this until it has been shifted to another forum, which I have asked the mentor to do.
     
  6. Jan 15, 2008 #5

    Doc Al

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    Is this the only data you took? If so, I don't see how you can deduce anything about the mass of the pole. Ask yourself: Is the pole uniform? Where is its center of mass? What if no weights were hung from the bottom hooks, does it still balance?

    (FYI: Even if you're totally stuck, it's a good idea to explain what you know and how you are thinking.)
     
  7. Jan 18, 2008 #6
    to answer your quesiton, we are to asume that to the ability we can measure the pole is uniform. We limited to what we were allowed to measure.
    my thoughts are that it has something to do with the torque presented by the two masses and the different lengths from the center. maybe this will help
     
  8. Jan 18, 2008 #7

    Doc Al

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    So, before you hung any weights from the pole it was in rotational equilibrium. Right?

    If so, hanging weights on it won't tell you anything about the weight of the pole, since its weight contributes no torque about the support.

    (Now if you hung the pole from a hook that was off to the side and not at the center, then you could figure out its weight by hanging other weights from it.)
     
  9. Jan 18, 2008 #8

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    Actually, the given torques don't balance about the centre. (User shvnchizms, have you checked this basic thing?) The rod is not probably uniform, or hung off-centre. Need more info.

    What did the OP mean by the two hooks on the bottom ? I presume he means on both the sides of the centre?
     
  10. Jan 18, 2008 #9

    Doc Al

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    Yes, I found the data suspect also. But first things first. :wink:
     
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