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Homework Help: Fluids (Find the tension)

  1. Nov 20, 2004 #1
    I could use some help on what formula or combination of formulas I am missing.
    An iron (p = 7860 kg/m3) cube measuring 15.4 cm on a side is suspended from a wire. Find the tension in the wire when the block is completely submerged in water (p = 1000 kg/m3). The answer is 246 N

    1) I have tried first to find the volume, v=m/p or v*p=m 3652*7860=28706795.04 , (3652=v, 15.4 cubed)
    2) I tried then to find F of the block with, Fb=(3562)(1000)(9.81)=35789600
    3) I then used both numbers in a final eq. FT=mg-Fb (28706795)(9.81)-(35789600)= to big of number

    Thanks for the help.
  2. jcsd
  3. Nov 20, 2004 #2


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    Note that the density is given in [tex]kg/m^3[/tex], but the side length is given in [tex]cm[/tex]. So be careful not to mix units.

    The volume of a cube 15.4 cm on a side is of course 15.4^3 or 3652.264 [tex]cm^3[/tex]. 1 m = 100 cm, so the volume, [tex]V[/tex], = 0.0036523 m^3.

    The tension,T, in the wire, which is simply the gravitational force exerted on the block, is given by,

    T = (m(block) - m(water))g.


    Note that the block is completely submerged so it displaces an equal volume of water.
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