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Help with a pipe, water, continuity and Bernoullie

  1. Feb 16, 2008 #1

    TFM

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    [SOLVED] Help with a pipe, water, continuity and Bernoullie

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

    At one point in a pipeline the water's speed is: 3.0 m/s,
    the gauge pressure is: 5.0*10^4.

    Find the gauge pressure at a second point in the line, 11m lower than the first, if the pipe diameter at the second point is twice that at the first.


    2. Relevant equations

    Bernoullies Equation: P1 + density*g*y1 + 0.5*density*v1^2 = P2 + density*g*y2 + 0.5*density*v2^2

    Continuity Equation: A1V1 = A2V2

    3. The attempt at a solution

    I work out the speed of v2 using continuity, with area-1 being pi*r^2, Area-2 being pi*4r^2,
    This makes the speed 3/4 m/s

    I themn put all known variables into Bernoullies equation, which gives me a negative pressure! (-53581.25) this cannot be right?

    Any Ideas/Suggestions

    TFM
     
  2. jcsd
  3. Feb 16, 2008 #2

    Doc Al

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    Staff: Mentor

    Looks good.

    No, it can't. What did you put for y1 & y2?
     
  4. Feb 16, 2008 #3

    TFM

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    I first put y1 as 0, y2 as 11, then tried y1 as 1, y2 as 12

    It should be y2 = -11, shouldn't it? -goves an answer of 162018.75
     
  5. Feb 16, 2008 #4

    Doc Al

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    Staff: Mentor

    Yep.
     
  6. Feb 16, 2008 #5

    TFM

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    162018 was the right answer:smile:

    Thanks

    TFM
     
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