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Homework Help: Tension in string of object suspended in liquid

  1. Jul 18, 2010 #1
    1. The problem statement, all variables and given/known data

    an aluminum object with volume 100 cm^3 and density 2700 kg/m^3 is suspended in ethyl alcohol (density 790) by a string. what's the tension in the string?



    2. Relevant equations

    Fy = Fb + T - mg
    T = mg - Fb
    Fb = p_f * v_f * g = p_o * v_o * g

    3. The attempt at a solution

    =(p_o - p_f)p_f*v_o*g

    =(2700 - 790)(790)(1*10^-4)(9.81)

    Thanks!
     
  2. jcsd
  3. Jul 18, 2010 #2

    mgb_phys

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    Not quite, check the units.

    It might be simpler if you worked in some numbers.
    Tension = weight down - boyancy up
    mass = density * volume = 2700 kg/m^3 * 0.0001 m^3 = 0.27 kg
    boyancy = weight of fluid = 790 kg/m^3 * 0.0001m^3 = 0.079 kg

    So effective mass of block (0.27-0.079) = 0.191kg,
    weight = 0.1919kg * 9.81 Newtons
     
  4. Jul 18, 2010 #3
    i figured out what i did wrong. your way was much easier, thanks :)
     
  5. Jul 18, 2010 #4

    mgb_phys

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    Useful tip always put the units in your calculation or rearrangment of equations - it makes it much easier to spot any little 'opps'

    Eg. to get mass from density.
    mass = density * volume
    kg = kg m^-3 * m^3 = kg !
     
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