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Calaculate Mass of Cube submerged in oil and water

  1. Feb 13, 2008 #1

    TFM

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    [SOLVED] Calaculate Mass of Cube submerged in oil and water

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

    A cube is submerged in water and oil the oil is on top, and forms a layer 10cm thick. the water is underneath and forms another layer 10cm thick. the cube has sides of 10.2cm, and has 2.3cm below the oil water line. the density of the oil is 790kg. I have worked out already the force on the top of the cube is 163 Pa, and on the bottom 1000 Pa. How do you work out the mass of the cube?

    2. Relevant equations

    Pressure = Force/Area

    Archimedes: The upward force is equal to the weight of fluid displaced by the block (?)

    3. The attempt at a solution

    Any help would be appreciated

    TFM
     
  2. jcsd
  3. Feb 14, 2008 #2

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    Use Archimedes' separately for the two parts submerged in the two liquids and add the buoyant forces to balance the weight. Hence, find mass. No need to find pressure.
     
  4. Feb 14, 2008 #3

    TFM

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    Am I doing simetghing wrong? I am doing:

    F = p*A
    A = 0.102^2 = 0.0104

    P top = 163
    Pressure Bottom = 1000

    F top:1.696
    F bottom = 10.404
    Sum F = 12.10

    using F = mg,

    m = 1.23kg

    But it is wrong?

    TFM
     
  5. Feb 14, 2008 #4

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    Show how you got these two.
     
  6. Feb 14, 2008 #5

    TFM

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    Therehave been stated as being right, because I am using mastering physics, and have previously calculated them and got them marked correct.

    I used P = denstiy*g*height,
    Density Oil = 790 (stated in question)
    height I calculated as being 2.1cm (0.021m)

    Works out as 163 Pa (Did same thing for Water at the bottom, but used the total pressure = pressure caused by the oil on top + the pressure from the water layer from the join to the bottom of the cube.)

    The Pressures are gauge pressures, by the way, if that helps?

    TFM
     
    Last edited: Feb 14, 2008
  7. Feb 14, 2008 #6

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    Why have you added the forces? Net force acting upward should be F_bottom - F_top.

    I still recommend that you learn how to solve using the method I had told you right in the beginning. If the body given is not a cube or parallelepiped, then you won't be able to do it using your way.
     
  8. Feb 14, 2008 #7

    TFM

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    Thanks. Finally got the right answer (0.889kg)

    TFM
     
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