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Homework Help: Multi tube pressure help

  1. Jan 21, 2004 #1
    See the picture.
    1 atm = 101300 Pa = 101300 N/m2
    density of water = 1000 kg/m3

    The tube is filled with water. A1 = 0.05m^2 and A2 = 0.08m^2. Two pistons apply different forces to the water in the tube so that the water in the right side of the tube is a height h = 0.43m above the height of the water in the left side of the tube. If F2 = 138 N what is F1?
    I know general equation is F/A equals pressure.

    Well, I wanted to do F1/A1=F2/A2... but the fact that both sides of the tube are not level gives me a problem.

    It's probably a simple concept, but I'm not seeing it. HOw do I set it up to account for the height=.43m factor?

    Attached Files:

  2. jcsd
  3. Jan 21, 2004 #2

    Chi Meson

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    The pressure is equal in both tubes at exactly the same height. THe pressure in the higher tube decreases according to the static fluid pressure formula, P=(rho)gh .

    So the pressure at the surface of the right tube is (rho)gh less than the other.
  4. Jan 21, 2004 #3

    Doc Al

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    Staff: Mentor

    The pressure in the fluid is the same at all points with the same height. So... set the pressure at the left side equal to the pressure on the right at depth h. Make sense?
  5. Jan 22, 2004 #4


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    What both Rockdog and Doc Al are saying is that the total force on the right (higher) side is F2+ the weight of the additional water. Since the additional height is h and the area is A2 the volume is hA2 and the weight is that times g times the density of water (which I'll call δ): The total Force on the right is F2+ gδhA2. The pressure is then F2/A2+ gδh and that must equal F1/A1.
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