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How does Archimedes' Principle work?

  1. Jan 30, 2012 #1
    the Archimedes' Principle states that the weight of the fluid displaced is equal to the buoyant force. why/how does that happen?
    Last edited: Jan 30, 2012
  2. jcsd
  3. Jan 30, 2012 #2
    The fluid is trying to fall below you, and has to push you up and out of the way to do so.
  4. Jan 30, 2012 #3
    Imagine a container of water and picture a volume of the water at some depth in the container. It might help to imagine this volume of water in a plastic bag (ignore the weight of the bag...it is only to help your imagination).
    This volume of water will not be sinking or rising so the effect of the rest of the water in the container is to provide a force upwards equal to the weight of the water in the plastic bag.
    Now replace the plastic bag of water with an identical volume plastic bag of steel. The water in the container continues to provide an upward force equal to the weight of water that was originally in place. Upward force on steel equals upwards force on weight of water that steel has replaced.
    No equations, no complicated maths.... hope it helps you cope with any equations and maths that you may meet.
  5. Jan 30, 2012 #4
    but if that happens, then the diagram on the right has buoyancy=0, because the liquid displaced has been removed.

    Attached Files:

    • jihu.png
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    Last edited: Jan 30, 2012
  6. Jan 30, 2012 #5
    that's cool, but i have already understood how buoyancy is the same for equal displacement of the volume of a fluid. i wanted to know what the displacement of the volume of a fluid has to do with buoyancy and how the mechanism works.
  7. Jan 30, 2012 #6
    The only extra that I can add involves talking about pressure due to a depth of liquid.
    If the top of your container is h1 below the surface then the pressure on the top of the container is P = h1ρg and the force on the top of the container is P x A = h1ρgA
    Similarly the force on the bottom is h2ρg if the bottom is at a depth of h2.
    The upwards force is therefore greater than the downwards for by (h2-h1)ρgA
    but this is the weight of the liquid that would occupy the same volume as the oject.
    So the upthrust = weight of displaced liquid.....(it does not matter whether the displaced liquid came out of the container or not)
    I hope this adds to the explanation for you
  8. Jan 30, 2012 #7
    thanks you, that answered my question :)
  9. Jan 31, 2012 #8
    In that diagram the ball still has water above it. This water is attracted towards the ground.
  10. Jan 31, 2012 #9
    oh. right
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