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Homework Help: Buoancy force question

  1. Mar 5, 2010 #1
    1. The problem statement, all variables and given/known data
    A cube made from iron has sides equal to 20cm/side. The density of iron is 7860kg/m^3. The cube is completely submerged in water; the density of water is 1000kg/m^3. What is the buoancy force acting on the cube.

    2. Relevant equations
    W(DL)= (D(L))(V)(g)

    3. The attempt at a solution
    W(DL)=(1000kg/m^3)(8000cm^3)(10m/s^2) =8x10^7 Neutons
    Im not sure if this answer is correct ????
  2. jcsd
  3. Mar 5, 2010 #2


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    Science Advisor
    Homework Helper

    Take care of your dimensions. You're using the density in kg per meter^3, but volume in cm^3.
    1 Newton is 1 [itex]kg m s^{-2}[/itex] You can get the right units from there.
    When in doubt, always use SI-units.
  4. Mar 5, 2010 #3


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    Homework Helper

    Force of buoyancy = weight of the water displaced by the cube. How can that small cube of water weight 8x10^7 newtons?

    Think, what is your weight? less or more than 8x10^7 N? Your average density is also very close to 1000 kg/m^3. Are you bigger or smaller than that 20x20x20 cm^3 cube?

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