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Homework Help: Cubical block buoyancy

  1. Dec 11, 2013 #1
    1. The problem statement, all variables and given/known data

    A cubical block of density ρb with sides lengths L floats in a liquid of greater density ρL. (A) what fraction of the blocks volume is above the surface of the liquid?
    (b) the liquid is denser than water (density ρw) and does not mix with it. If water is poured on the surface of the liquid, how deep must the water layer be so that the water surface just rises to the top of the block? Express your answer in terms of L, ρb, ρL and ρw. (c) Find the depth of the water layer in (b) if the liquid is mercury, the block is made of iron, and its side length is 10 cm.

    2. Relevant equations


    3. The attempt at a solution
    I really need to know the answers to these questions, because the idea is pretty simple, but I'm simply stuck!! For (a), I thought of the following: ρb*L3L*L2*(L-x) where x is the part which is above water. How can I find a fraction??
    For (b), my mind is really freaking out. I understand that there is a surface of water ON top of the liquid ρL. If the water layer rises on top of the block, the height of it will be (L-x) while the height of the liquid with density ρL will be x itself. Can anybody please help me out?? I would really appreciate your help!
  2. jcsd
  3. Dec 11, 2013 #2


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    For a), what (in terms of L and x) is the volume of the block above the surface? What fraction is that of the block's total volume?
    (For part (a), you don't really need to assume the block floats level - it could float at any angle. You can just use volumes rather than lengths.)
    Yes, but a different x now. What weight of the denser liquid is displaced by the block now? What weight of water is displaced by the block? What equation can you write connecting those with the weight of the block?
  4. Dec 12, 2013 #3
    Hmm. I do not understand how I can give the fraction for the volumes. Can anybody show this idea to me? Please???
  5. Dec 12, 2013 #4


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    Fr part (a), you are taking the block to have side L, and a length x of that is above the water.
    What volume of the block is above the water?
    To get that as a fraction of the whole volume, you divide that by the whole volume, L3.
    You have an equation which gives you x as a function of L and the densities.
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