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Homework Help: Weight in water

  1. Aug 17, 2009 #1
    1. The problem statement, all variables and given/known data
    An object weighing 341 N in air is immersed in water after being tied to a string connected to a force scale. The scale now reads 307 N. Calculate the density of the object.

    2. Relevant equations
    Fb=(density of the surrounding liquid)(V)(g)

    3. The attempt at a solution
    Fb is difference in the forces, 341N-307N = 34N
    v = ((initial weight)-(submerged weight))/9.8 = 3.47
    mass = 341N/9.8

    So Density = m / v

    Density therefore = 10.3kg/m^3

    Its wrong, i have no idea why, probably some unit conversion somewhere.

  2. jcsd
  3. Aug 17, 2009 #2


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    The 34N that you found is the buoyant force, i.e. the weight of the displaced water. Can you find the mass of the displaced water and then the volume of the displaced water? This volume is the object's volume.
  4. Aug 17, 2009 #3
    The bouyancy force is B = (Density of liquid)(Volume of the object) * g.

    V is not the difference in mass (which is what I think you were trying to do), it's the volume of the object.

    So you know B and g, and the density of water is fairly constant so you can solve for the volume.

    Density as you said is mass divided by volume, and you correctly found the mass, so you just need to find the volume.
  5. Aug 17, 2009 #4
    Isn't that what i did? B = Density * V * g and like you said water is fairly constant, so V = B/g
    which is 34N/9.8 = 3.47 which is the volume of the object, right?
    So now i can do D = m/v for the density of the object given m of object is 341/9.8.

    And i just confused myself, we're ignoring the density of water cause its "fairly constant" right?
  6. Aug 17, 2009 #5


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    You need to include the density of the water in your volume calculation.
  7. Aug 17, 2009 #6
    Ok, now ill sound just plain stupid.
    How do i find the density of water?
  8. Aug 17, 2009 #7


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    You can look it up. Its normally 1000kg per cubic metre.
  9. Aug 17, 2009 #8
    Ahh i see what i did thank you all for the help
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