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Homework Help: Fluid mechanics calculating force needed to lift submerged disc

  1. Feb 20, 2012 #1
    1. The problem statement, all variables and given/known data
    The problem:

    There is an illustration and the question is to find the force F to lift a concrete block gate if the the concrete weighs 160 lb/ft3. The block is 3' in diameter and 1' thick. It is in a tank of fresh water 15 ft down. the specific weight of the fresh water is 62.4 lb/ft3 and specific weight of the sea water on the other side is 64 lb/ft3

    Specific weight fresh water:62.4 lbs/ft3
    Specific weight salt water 64 lb/ft3
    concrete: 160 lb/ft3, 3' diameterx1' thick

    2. Relevant equations
    equations I see:

    3. The attempt at a solution
    Steps I see so far
    1. Find area of the disc
    2. calculate the Volume of the disc
    3. Calculate weight of disc
    4. Sum of forces acting on disc
    -weight acting down
    -Calculate force needed to act upward

    -Does this use Bernoulli's equation?

    Attached Files:

  2. jcsd
  3. Feb 20, 2012 #2
    If the fluid is not moving you do not need to be concerned with Bernoulli's equation.

    What about buoyancy? You did not mention it.
  4. Feb 20, 2012 #3
    Looking at the problem again I did forget bouyant force so....
    Weight=Bouyant force when an object floats.... Analysis of the system bernoullis eqations at three points
    1. top of sea water
    2. At concrete disc
    3. surface of freshwater

    Would I calculate the pressures at the points to figure the impact on the Concrete disc and then factor that force needed to lift the disc is F(seawater)+Bouyant force.
  5. Feb 20, 2012 #4
    When the water is not flowing, there is no velocity so all you have is depth and pressures. You can think of Bernoulli's equation because two terms remain if you wish.

    The buoyant force is the amount of force the liquid exerts on an object in the upward direction. It is created by pressure differences applied to the surface area of an object.

    Do a force balance on the concrete. You have several forces to consider.

    1. Its weight
    2. Fresh water force
    3. Salt water force
    4. Force to dislodge it.

    The sides don't matter because they are vertical so the pressure only causes forces perpendicular to the direction you are considering.
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