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Archimedes (Buoyant force) Not EZ!

  1. Jun 21, 2007 #1
    I tryied but failed miserabely, i need to prove this formula!

    Archimedes (Buoyant force)

    Lets say that a rock is attatched to a string, the string has a tention of 5.0N out of the water, and has a tention of 3.2N submerged in water. Now to find the density of the rock, the formula used is:

    p rock=(5.0/5.0-3.2)p water

    To explain...

    density of rock=(wt in air/ wt in air-wt in water)*density of water

    Now i need to prove this formula, i would apreciate any help, thanks


    What i know:
    Density of water is 1000kg/m^2

    I tried many different ways but never got a definite answer
    Last edited: Jun 21, 2007
  2. jcsd
  3. Jun 21, 2007 #2


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    Homework Helper
    Gold Member

    You can start by drawing the freebody diagram for the rock, and calculate the net force on the rock in and out of the water.
  4. Jun 21, 2007 #3
    I know all that, i know the density of the rock and everything, i just need to prove the formula (show why it works)
  5. Jun 21, 2007 #4
    i need to prove

    density of rock=(wt in air/ wt in air-wt in water)*density of water
  6. Jun 21, 2007 #5
    Start from lost weight in water = buoyant force of water = ......
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