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Homework Help: Fluid mechanics cork in water

  1. Dec 15, 2012 #1
    1. The problem statement, all variables and given/known data
    A cork is held at the bottom of a bucket of
    water by a piece of string. The actual depth
    of the cork is 0.713786 m below the surface of
    the water.

    If the density of the cork is 157 kg/m3
    and the volume of the cork is 3 cm3
    , then what is the tension in the string? The acceleration
    of gravity is 9.8 m/s
    . Assume the density of
    water is 1000 kg/m3

    2. Relevant equations
    d= m/v Fb= weight of displaced fluid

    3. The attempt at a solution
    The tension should be the force required to keep it in equilibrium...
    using d=m/v, into dv = m (1000)(0.03) = 30 kg (mass of displaced water)(9.8)
    weight of displaced water = Fb = 294 N
    using dv = m again for the cork. (157)(0.03) = 4.71 kg (9.8) = 46.158 N

    294 - 46.158 = 247.842 N .
    Its a multiple choice answer and the choices are either that number but the decimals in the wrong place (all the answers are smaller), or 0.0123921 N.
    maybe I'm supposed to use the depth and P=pgh somehow if that other numbers right, but i'm not sure how it would apply to this problem.
  2. jcsd
  3. Dec 15, 2012 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Units, Units, Units!!!

    3 cm^3 is not the same as 0.03 m^3

    You are essentially saying that 3 cc of water has a mass of 30 kg!
  4. Dec 15, 2012 #3
    that makes sense... so it would be like (0.03)^3 then i guess
  5. Dec 15, 2012 #4


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    Gold Member

    Looks like the problem setter didn't think of this particular mistake, or there would have been a choice to match.
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