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Homework Help: Simple Pressure\Speed question

  1. Oct 14, 2012 #1
    1. The problem statement, all variables and given/known data

    Water is coming out of a water cannon at 25m/s, what is the pressure inside the cannon?

    2. Relevant equations
    Bernoulli's (modifiedd): v = √(2gh) -> h = v^2/2g
    Prssure(liquid) : P = ρgh

    3. The attempt at a solution

    Well, I wasn't too sure how a water cannon works, so I "made one" that is just a giant vat with a lot of water, and a hole at the bottom.

    I figured that for the water to come out at 25m/s, the height would have to be ~31m.
    h = (25^2)/2g

    So then I said, Pressure must be equal to ρgh, which is 1000 * 9.8 * 31m = 303,800Pa

    But the answer the book gives says its "4.125 x 10^5 Pa absolute."

    Any idea why my solution is incorrect?
  2. jcsd
  3. Oct 14, 2012 #2
    Absolute pressure? Did You take into consideration atmospheric pressure?
  4. Oct 14, 2012 #3
    Oh jeeze, I guess that would give me the ~100000 Pascals I need for the answer, thanks!
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