1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: 2 problems of fluids help

  1. Dec 3, 2004 #1
    16. The normal atmosphercic pressure is 1.013 x 10^5. A storm causes that the height of the barometer of mercury reduce 20.0 mm form the normal height. What is the atmosferic preasurre ? ( the density of mercury is 13.59 g/cm3

    50. Out of a fire extingisher comes out water under air presurre. What manometric pressure is required in the tank for the water burst to have a speed of 30 m/s when the water level is 0.5 m below the water pump.

    as hard as i try i really cant solve is problems can anyone help me???
  2. jcsd
  3. Dec 3, 2004 #2
    i got the first one , can anyone help me with the other one?
  4. Dec 4, 2004 #3

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    It is not entirely clear on the question but it seems that the air pressure has to lift the water .5 m and send it out at 30 m/sec.

    Think of pressure as an energy density (energy/unit volume):
    [tex]P = \frac{\delta E}{\delta V}[/tex]

    In order to lift an element [itex]\delta m = \rho \delta v[/itex] of water h = .5 m. and accelerate it to v = 30 m/sec energy of:

    [tex]\delta E = \delta mgh + \frac{1}{2}\delta mv^2[/tex]

    is required. So an energy density of:

    [tex]\frac{\delta E}{\delta V} = \rho gh + \frac{1}{2}\rho v^2 = P[/tex]

    [tex]P = \rho (gh + \frac{1}{2}v^2)[/tex]

    [tex]P = 10^3 (9.8 \times .5 + .5 \times 900)[/tex]

    [tex]P = 4.55 \times 10^2 KPa[/tex] or about 4.55 atm. (4.55 bars)

    Last edited: Dec 4, 2004
  5. Dec 4, 2004 #4
    thanks andrew but i have another question

    so i did P = 10^3(9.8*0.5 +900*0.5) = and it gives me another answer diffrent that yours it gives me 4.55 x 10^5 half of what you got , why did u multiply it by two ??
  6. Dec 4, 2004 #5

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Just forgot to multiply by the .5. See edited reply above

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook