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Homework Help: Quick Question

  1. Dec 7, 2004 #1
    Can I use either of these formulae to find the desity of an object?

    a) ((Weightapparent/Forcebuoyant) + 1)(Densityfluid) = Densityobj

    b) (Weightobject/Forcebuoyant *Densityfluid) = Densityobj

  2. jcsd
  3. Dec 7, 2004 #2
    Archimedes law tells you the buoyance force is equal to the gravitational force on the displaced mass:

    [tex]F_{buoyance}=g \rho_{fluid} V_{displaced}[/tex]

    The apparent weight of the object can be found by evaluating the net downward force [itex]F_g-F_{buoyance}=g[m-\rho_{fluid} V_{displaced}][/itex] Where the term between brackets is the apparent weight.

    With [itex]\rho[/itex] density, [itex]m[/itex] mass [itex]V[/itex] volume and g the gravitational acceleration. With this knowledge you can probably work out the answer yourself!
  4. Dec 7, 2004 #3
    I still can't figure it out :mad:
  5. Dec 7, 2004 #4


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    What's to figure out than applying what Da-willem said??

    You may wanna write:
    [tex] \rho_{body}-\rho_{fluid}=\frac{F_g-F_{buoyance}}{g V_{displaced}}[/tex]
    From the previous equation,the density of the body is immediate.
  6. Dec 7, 2004 #5
    ok, so my original two formulas are correct then, right?
  7. Dec 7, 2004 #6


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    Yes,they are true.But why complicate with calculating buoyant forces,when u can simply say that:
    [tex] \rho_{object}=\frac{m_{object}}{V_{displaced}} [/tex]
    It's Archimede's law that states that,if the fluid is incompressible (Archimedes didn't think of compressible fluids),then the volume of the body is equal to the volume of the displaced fluid.Since u can easily determine one's body mass,the density is immediate.

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