Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: At what value of h does the flow stop?

  1. Oct 17, 2004 #1
    I am new here so I hope that someone may have some advice. I am having trouble with a homework question which is quite lengthy. -- A large tank of water has a hose connected to it as shown in the figure. The tank is sealed at the top and has compressed air between the water surface and the top. When the water height, h, has the value 3.50m, the absolute pressure p of the compressed air is 4.20x10^5 Pa. Assume that the air above the water expands at a constant temperature, and take the atmospheric pressure to be 1.00x10^5 Pa. a)What is the speed with which the water flows out of the hose when h=3.50m? b)As water flows out of the tank, h decreases. Calculate the speed of flow for h=3.00m and h=2.00m c)At what value of h does the flow stop?-- The figure shows a tank of height=4.00m and the water level is lower than 4.00m giving the air gap. The hose is on the side in a "Z" shape with the top of the "Z" being 1.00m from ground level. I don't know if anyone can help, but I would appreciate the input.
  2. jcsd
  3. Oct 18, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You will have to show us what you've done so far. (Read the sticky post at the top of the forum).
  4. Oct 18, 2004 #3
    i believe that i figured out a) so for b) i was trying to determine the pressure at 3.00m. i used PV=nRT for 3.00m and PV=nRT for 3.50m and equated them. The nRTcancel and i get PV=PV. The tank is a cylinder so i said PAh=PAh. The A's cancel. P1h1=P2h2 with P1=4.20x10^5Pa, h1=3.50m, h2=3.00m solve for P2. I have done something wrong because this gives an increase in pressure when it should decrease. Anyone have some help for me?
  5. Oct 18, 2004 #4
    Why not use your knowledge of calculus?
  6. Oct 18, 2004 #5
    i think i just figured it out. thanks to anyone who considered the problem
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook