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Homework Help: Tension in the string of a submerged object

  1. Jun 12, 2009 #1
    1. The problem statement, all variables and given/known data
    An aluminum object suspended from a string with density 2700 kg/m^3 is submerged into ethyl alcohol.

    what is the tension in the string? V=142 cm^3

    2. Relevant equations
    vsub/vobj=(ro)object/(ro)ethyl alcohol

    3. The attempt at a solution
    I tried to multiply the volume 142 cm^3 (.000142 m^3) by the density 2700 kg/m^3 to get the mass. Then I multiplied the mass by gravity since Archimedes' principle tells us that Ethyl Alcohol pushes with a force equal to the weight. But that is wrong for some reason. Please help me. Thank you.
  2. jcsd
  3. Jun 12, 2009 #2


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    Homework Helper

    Welcome to PF.

    What is the weight of the displaced ethyl alcohol?

    Wouldn't you subtract it from the free weight of the object?
  4. Jun 12, 2009 #3
    That is correct. But why would you subtract the weight of the liquid from the weight of the object? Is it because the weight I calculated was just the apparent weight?
  5. Jun 12, 2009 #4

    Doc Al

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    Staff: Mentor

    You calculated the mass of the aluminum object, which you'll need but is not the final answer. Archimedes' principle says that the liquid pushes up with a buoyant force equal to the weight of the displaced liquid, not the weight of the object.
  6. Nov 26, 2010 #5
    I have just tried the quesiton..just gonna list what i did..
    Fnet = Ftension+ Fbuoyancy-Fgravity=0
    Ftension = Fgravity-Fbuoyancy
    => m(object)g - P(fluid)*V(fluid)*g
    =>P(object)*V(object)*g - P(fluid)*v(fluid)*g
    In my case the aluminum has same density,2700kg/m^3..but i has the volum of 100cm^3
    and for completely submerged object V(fluid)= V(object)...which is 1 m^3 in my case
    The average value of density for ethyl alcohol i found online is 790 kg/m^2
    After plug in everythign that is given and know..i got an answer of 1.87 (round of to 1.9 since 2 sig fig)
    Hope that help..
    P.S this quesiton is from my mastering physics as well..and i am in first year taking introductory physics...i am using physics for scientists and engineers 2nd edition..u can find some helpful informations from chapter 15(pg456) if you didn't get what i wrote..
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