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Homework Help: Fluid, pressure and weight question

  1. Mar 16, 2009 #1


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    Gold Member

    Hi PF,
    I think I solved well the problem but I don't know what's going on and I thought about it last night and I am at a loss on this...
    1. The problem statement, all variables and given/known data
    Consider a closed cylinder whose height is H and with radius R. On its top there is a cylinder of height H and radius [tex]\frac{R}{100}[/tex] such that we can fill the 2 cylinders with liquid via the cylinder on the top of the other. (They are connected such that the liquid can flow between the cylinders).
    Compare the weight of the liquid with the hydrostatic force supported by the base of the first cylinder.

    2. Relevant equations
    [tex]\Delta F=P \Delta A[/tex]
    [tex]P=\rho gH[/tex]

    3. The attempt at a solution

    I found out that the weight of the liquid is [tex]\frac{5001 \rho \pi R^2 gH}{5000}[/tex] while the force supported by the base of the first cylinder is [tex]\frac{5000.5 \rho \pi R^2 gH}{5000}[/tex]. I find this incredible that the ground doesn't have to support all the weight of the liquid... I don't know what's going on. The problem is of course under the column of water formed by the second cylinder. It seems that its weight is greater than the force needed to support it. Maybe molecules are going upward there and so forming a flow... I really want to know!
    Last edited: Mar 16, 2009
  2. jcsd
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