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Homework Help: Fluids involving Forces

  1. Dec 5, 2005 #1
    This one is a doozy . . .

    A fresh water pond that is 15.7 m deep is contained on one side by a cliff. The water has eroded a nearly horizontal "tube" through a bed of limestone, which allows the water to emerge on the other side of the cliff. If the "tube" has a diameter of 4.05 cm, and is located 5.6 m below the surface of the pond, what is the frictional force between the "tube" wall and a rock that is blocking the exit?

    I've spent a bit too long on this one, and I am getting nowhere . . . any assistane would be greatly appreciated. The sooner the better as well lol. Thanks for your help in advance.
  2. jcsd
  3. Dec 5, 2005 #2
    actually I had the same question as well.

    I'm wondering if I am missing a Force in my force diagram.

    I used F = PA
    And then the equal and opposite reaction to the force of the water pushing on the rock would be the friction between the rock and the wall going in a direction opposite to the direction of motion.
    and where P = Patm + Dgh, where D= density of the fluid and Patm is the atmospheric pressure pushing down on the water.
    Is that what you did?
  4. Dec 5, 2005 #3


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    Homework Helper

    Use the summation of forces Fnet = 0.

    Your rock is acted on by
    1. friction
    2. air pressure - side 1
    3. water pressure - side 2

    Since, the hole is relatively small, we can make the calculations simple with not statics of bodies involved. The pressure of the water on side 2 of the rock is equal to:

    [tex] p_w = \rho g h + p_{atm}[/tex]

    Using F = pA, the answer shouldn't be far off.
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