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Pressure, Buoyant Force problem

  1. Nov 13, 2011 #1
    1. The problem statement, all variables and given/known data
    A solid cube of unkown material is floating upright in water so that 1/4 of the cube is above the surface of the water. What is the density of the material?


    2. Relevant equations
    Density=m/v
    F=mg
    Fb=density of fluid times volume of the fluid times g
    P=density times height times g
    3. The attempt at a solution
    Since the cube is floating upright in the water I think that means that the force of weight is equal to the buoyant force.
    P=density times h times g
    P=1000 kg/m^3 times 1/4 times 9.80 which equals 2450 Pa
    Then I used F=PA
    F=2450 Pa times 1/4 which equals 612.5 N
    Then I used F=mg
    m=F/g
    m=612.5 N/9.80 m/s^2
    m=62.5 kg
    I dont know what 2 do from there and I think one of my step is wrong because I multiplied the area by 1/4 but I dont think that's right can someone please help me and explain 2 me how to solve this problem
     
  2. jcsd
  3. Nov 13, 2011 #2
    (Density of the object / density of the fluid) x 100 = Prencentage of object's volume in the fluid
     
  4. Nov 13, 2011 #3
    But when I do that I get the following:
    b/1000 kg/m^3 times 100=3/4
    100b/1000=3/4
    1b/10=3/4
    b=7.5 and b is the density of the object but on my sheet it says the answer is 750 kg/m^3 and its almost like my answer but the decimal is moved over and idk y. Maybe my work was wrong or something can u explain ?
     
  5. Nov 13, 2011 #4
    But when I do that I get the following:
    b/1000 kg/m^3 times 100=3/4
    100b/1000=3/4
    1b/10=3/4
    b=7.5 and b is the density of the object but on my sheet it says the answer is 750 kg/m^3 and its almost like my answer but the decimal is moved over and idk y. Maybe my work was wrong or something can u explain ?
     
  6. Nov 14, 2011 #5
    3/4=0.75 is not the Precentage dude, it's 75%
     
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