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HW about presure and wight I need your consulting

  1. Aug 25, 2008 #1
    1. The problem statement, all variables and given/known data
    a U-tube with cross-sectional area A and partially filled with oil of density S. A solid cylinder, which fits the tube tightly but can slide without friction, is placed in the right arm. The system is in equilibrium. Find the weight of the cylinder in terms of g, h, A, L, and S.

    2. Relevant equations
    I'm using these relationship ;where F is the force of the wight (F=W) & h is hight
    delta(P) = S*g*delta(h)

    3. The attempt at a solution

    look at my solving and also the figure of question in attachment and tell me what is the wrong in my steps ? why h doesn't appears in the last result?
    and thank you very much for your helping before

    Attached Files:

  2. jcsd
  3. Aug 26, 2008 #2
    point A and point B I take them in same level where A under the cylinder directly and B in versus side
    Hi misters; any body can help me, please
  4. Aug 26, 2008 #3


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    Homework Helper

    To be in equilibrium won't the weight of the cylinder equal the weight of the volume of water of height L on the other side?

    Volume is Area times L on the left. Density of water is p.
    So isn't that simply W = pgAL
  5. Aug 26, 2008 #4

    why (h) doesn't appears in the last result?

    yah I understand you ..
    but your result or the weight of cylinder should be depend on all the variables as the Q.

    [Find the weight of the cylinder in terms of g, h, A, L, and S.(S or p is density)]
    pleas can you explain >>>

    thank you Mr.LowlyPion
  6. Aug 26, 2008 #5


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    Homework Helper

    I took that to mean to express the answer in the unknowns given by the drawing. The answer by inspection of the drawing looks to be independent of h.

    Think of it this way. What property of the weight of the cylinder has anything to do with h? L on the other hand defines the volume of oil required to be displaced to hold the cylinder in equilibrium. (You remember Archimedes don't you?)

    In your solution you used area in the wrong manner on the weight. It's the weight of the cylinder divided by A that establishes pressure.

    What you can say about h is that the solution is only valid for h greater than the diameter of the tube - A/2pi*U2
  7. Aug 26, 2008 #6
    yah that is clearly ... thank you Mr.LowlyPion for your helping
    there are small thing also , from where you find this A/2pi*U^2 (U?) ?
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