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Homework Help: Bernoulli/ Continuity Eq'n problem

  1. Aug 4, 2012 #1
    First of all, thanks to all the PF mentors out here, especially TSny and pgardn, who have made physics doable and are helping me accomplish my dreams! Even when putting in the work its not easy to get all this stuff!

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

    A large water tank has an inlet pipe and an outlet pipe. The inlet pipe has a diameter of 3 cm and is 1 m above the bottom of the tank. The outlet pipe has a diameter of 9 cm and is 7 m above the bottom of the tank. A volume of 0.6 m3 of water enters the tank every three minutes at a gauge pressure of 1 atm.

    (a) What is the velocity of the water in the outlet pipe?

    2. Relevant equations


    DV/Dt=AV (for both)

    3. The attempt at a solution

    I've done three practice problems, and gotten them all right. The difference between those and this one, is that the volume of water enters over three minutes, instead of one. So I think that's part of where I'm messing up.

    My method:

    Knowing DV/Dt = Av, I plugged and chugged:

    .6/180=(.045^2)pi*v1=> v1=.524 m/s. This exact method worked before, but for this problem, the answer is .589 m/s. Does anyone see where I messed up?
  2. jcsd
  3. Aug 4, 2012 #2
    Apparently this is a tough one :)
  4. Aug 4, 2012 #3


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    Hello Jamesbiomed. If the problem stated that the volume flow rate in the outlet pipe is the same as the volume flow rate in the inlet pipe then I think your answer would be correct. But it could be that the amount of water in the tank is changing. Then the outflow rate would not match the inflow rate. It doesn't seem to me that there is enough information given. I guess there are additional questions being asked in the problem besides part (a). Perhaps those additional parts give some hints as to any assumptions that you are supposed to make.
  5. Aug 5, 2012 #4
    Hey TSny! The only other part is a follow up:

    (b) What is the gauge pressure in the outlet pipe?

    So the gauge pressure will be different (which would be true whether the volume flow rate is the same or not I think)

    Like I said, in three practices, setting volume flow rate equal to AV gave me the correct answer. So I would guess it they mean the rate should be equal. Since I don't know initial "v" I don't see another way to go about it if not.
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