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Homework Help: Time it takes to fill a pool.

  1. Nov 27, 2008 #1
    1. The problem statement, all variables and given/known data

    To fill a child's inflatable wading pool you use a garden hose with a diameter of 2.8 cm. Water flows from this hose with a speed of 1.1 m/s. How long will it take to fill the pool to a depth of 32 cm if it is circular and has a diameter of 2.7 m?

    2. Relevant equations
    delta m=p1 A1 v1 delta t
    (p=density, A=area, v=velocity, t=time)

    A1 v1=A2 v2

    Q=A v

    idk if there is another equation that deals with time, i can't find any.

    3. The attempt at a solution
    since everything is dealing with water, i didn't really consider density.

    the amount of water needed:
    V= pi r^2 h=pi (1.35)^2 (.32)= 1.83 m^3

    the area of the hose is:
    A= pi r^2= pi (.014)^2= 6.16e-4 m^2

    Q=A v=6.16e-4 (1.1)=6.78e-4 m^3/s

    mass flow rate= .6776 kg/s

    i don't really know where to go from here...
    Last edited: Nov 27, 2008
  2. jcsd
  3. Nov 27, 2008 #2
    first, your volume of pool measurement is a little wrong. You've used the diameter as a radius in your equqtion.
    You've also made the sam mistake in calculating the cross sectional area of the hose.
    delta m over delta t will give you your mass flow rate. But you need your volumetric flow rate. so how do you turn mass in a volume (hint: pensity is required)

    keep trying and le tme know.
  4. Nov 27, 2008 #3
    my typing is terrible this morning, sorry for the spelling mistakes. going to get some more coffee :smile:
  5. Nov 27, 2008 #4
    i fixed the problems and hopefully they are right. i found the mass flow rate and also the volumetric flow rate. we haven't learned about pensity in our class yet...so there has to be a different way.
  6. Nov 27, 2008 #5
    i figured it out. thanks :)
  7. Nov 27, 2008 #6
    pensity was a spelling mistake for density, sorry. Glad you got it, no prob.
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