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Homework Help: Free Body Diagram Question

  1. Jan 17, 2010 #1
    Hey guys,

    Thanks for all the replies. I ended up having the test for this subject and the question was a bit different to what I was expecting. I have attached an image similar to that of the test question.

    Basically the question asked to explain what would happen to the angular acceleration (which is not constant) of the bar as the distance L increased.

    When I used the formulas they showed that as the distance L increased the moment the counterweight exerted on the trunnion pin increased. I know the formula: sum of moments=moment of inertia x angular acceleration. So that would mean the angular acceleration of the bar would increase?

    The system works on the basis that Wc is just heavy enough to overcome S & W, however with the buoyant weight there, the system will not move. As the buoyant chamber fills up with water the buoyant weight due to buoyancy forces lifts and the gate slowly begins to move.

    So my argument was, that even though the L length was increased and hence angular acceleration (which is not constant in the system) would increase the motion of the system would depend on the flow rate of water into the buoyancy chamber.

    Thus if the same flow rate was maintained, the buoyant weight would move upwards with the same velocity and thus the system would move with the speed/acceleration as before?

    So the class was divided 50/50…just wondering what you guys think?

    Attached Files:

  2. jcsd
  3. Jan 18, 2010 #2


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    Science Advisor
    Homework Helper

    Fill in your equation in paragraph 3, then solve for angular acceleration, alpha, to see what happens to alpha as a function of L.
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