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Homework Help: Simple buoyancy question ?

  1. Aug 29, 2010 #1
    1. The problem statement, all variables and given/known data

    A 700 g solid cube of edge length 10 cm floats in water. If the density of water is 10^3 kg/m^3, find the volume of the cube outside water.

    2. Relevant equations

    So the solution to this tells me that 'the weight of the cube should be balanced by the force of buoyancy and the force of buoyancy is equal to the weight of the water displaced'. And there is this other formula that says:

    Apparent Weight of object in water = Real Weight of object - Force of Buoyancy

    3. The attempt at a solution

    I don't understand. According to the formula the force of buoyancy is the real weight of the object minus its apparent weight in water, but the solution says that the force of buoyancy must be balanced by the weight of the object. Can someone explain this to me, and possibly tell me how I am supposed to deal with questions that involve buoyancy in general?

    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Aug 29, 2010 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What you need here are two things:
    (1) Archimedes's principle: The buoyant force equals the weight of the displaced fluid.
    (2) Conditions for equilibrium: Since the object is floating, the net force must be zero.

    There are only two forces on the object: Gravity (down) and the buoyant force (up).
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