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AP Physics Free-Response Practice Question #2

  1. Dec 5, 2007 #1
    1. The problem statement, all variables and given/known data
    A large tank, 25m in height and open at the top, is completely filled with saltwater (density 1025 kg/m^3). A small drain plug with a cross-sectional area of 4.0 X 10^-5 m^2 is located 5m from the bottom of the tank.

    The plug breaks loose from the tank, and water flows from the drain.

    (a) Calculate the force exerted by the water on the plug before the plug breaks free.

    (b) Calculate the speed of the water as it leaves the hole in side of the tank.

    (c) Calculate the volume flow rate of the water from the hole.

    2. Relevant equations

    I'm not sure...I used the continuity equations A1V1 = A2V2

    3. The attempt at a solution

    I calculated the volume of the water displaced and used the continuity equation and plugged in the numbers, but then I noticed that this was not the right way.
    Last edited: Dec 5, 2007
  2. jcsd
  3. Dec 5, 2007 #2


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    Start with part (a). How would you find the force on the plug?
  4. Dec 5, 2007 #3


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    You can change the density from being kg to N if you account for gravity:
    gravity per volume = density * gravity. the units look like this:
    N/m^3 = (kg/m^3) * (m/s^2)

    From there you can multiply by the depth to get the pressure
    N/m^2 = (N/m^3) * m

    Now that you have the pressure in N/m^2, you can multiply by the surface area of the plug to get the force.
    N = (N/m^2) * (m^2)

    Not exactly sure how to find speed, but I have a rough idea. It's probably wrong but it's wroth a shot...

    A little birdie suggested via report post that I delete this probably wrong response to avoid confusing the OP, who just wants help with his homework.
    Last edited by a moderator: Dec 5, 2007
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