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Homework Help: Pressure at points on a pipe question help

  1. Jun 11, 2017 #1
    1. The problem statement, all variables and given/known data
    Difficult pressure at pipes question.PNG

    2. Relevant equations
    AV = AV
    P + 1/2*p*v^2 + pgh = P + 1/2*p*v^2 + pgh

    3. The attempt at a solution
    For a.) ... divide 3 by 0.1 because of (AV = AV) = 30 ms-1. answer is right.
    For b.) im stuck on this one. I tried using the 2nd relevant equation above but i cant seem to find the answer and im sure i need to use that one. Heres my working out.

    Pressure A: P + (0.5 *1000 *3^2) = 5 *10^5... Therefore P = 495500 Pa
    Pressure B: P = 495500 + (0.5 *1000 *30^2) + (1000 *10 *1) = 955500 Pa
    Answer is 6.5 *10^4... (65000)

    For c.) point is located at the same height and same area as A, therefore pressures are equal = 5 * 10^5 Pa. Answer is right

    Any help would be greatly appreciated. Thanks in advance
  2. jcsd
  3. Jun 11, 2017 #2


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    You don't seem to understand the Bernoulli equation. P is the pressure at the given point. At A it is 5*10^5 Pa. The other terms are then added to this to get a quantity that is the same throughout the streamline.
  4. Jun 12, 2017 #3
    Hello sorry for the late reply.
    i think i see where i went wrong...
    Using your advice, i added the pressure change due to height and subtracted the pressure difference between the two speeds (as fluid speeds up pressure must drop)

    5*10^5 + (1000 *10) - ((0.5 * 1000 * 30^2) - (0.5 * 1000 * 3^2)) = 64500

    But im still 500 Pa off from the right answer, am i on the right track?
  5. Jun 12, 2017 #4


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    That's what I get. Looks like the answers have been rounded to 2 significant figures.
    The question statement is poor. It gives values like "1m2", "5x105Pa". Standard interpretation says these are only accurate to one decimal place. If you follow that through the calculation you find that the answer is indeterminate. E.g. just change the 3m/s to 3.1m/s and you get a much smaller answer. It should quote the values as 3.00m/s etc. to justify 2 sig figs in the answer.
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