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Cylinder submerged in water

  1. Dec 15, 2008 #1
    1. The problem statement, all variables and given/known data
    A 6.40 cm-tall cylinder floats in water with its axis perpendicular to the surface. The length of the cylinder above water is 1.00cm.

    2. Relevant equations
    What is the cylinder's mass density?

    3. The attempt at a solution
    Well, I am confused because I can use the equation for buoyancy to find the mass of the cylinder, but I don't know how to find its volume.
  2. jcsd
  3. Dec 15, 2008 #2


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    You don't need the radius of the cylinder to find the volume. The question only asks for its density. Just write out the equation using 'r' to denote the unknown radius and something will cancel out.
  4. Dec 15, 2008 #3
    so would i use the forumula F = rho* V * g?
  5. Dec 16, 2008 #4
    The way I would go about doing this problem--I don't know if it's the best way--is to say the cylinder has a mass m, then figure out the volume of water needed to equal this mass. You can then equate this to a formula for mass of the cylinder.
  6. Dec 16, 2008 #5


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    You still need to break V up in terms of r and h.
  7. Dec 16, 2008 #6


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    Weight of the cylinder= Area*6.4 cm*density*g
    Weight of the displaced water= Area*5.4*1000*g
    Now find the density.
  8. Jun 11, 2009 #7
    I have the same problem jsut different numbers..... I am confused on what to do. F=rho*v*g
    then after you cancel out you get
    F=mhg right? is that the force on the bottom?
  9. Jun 11, 2009 #8
    For a floating object the fraction of the volume of a floating object that is below the fluid surface is equal to the ratio of the density of the object to the density of the fluid:

    V-f/V-o=rho-o/rho-f as has been noted, the radius cancels, you know rho-f, you can solve directly for rho of the object.
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