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Buoyancy & Objects

  1. Jul 24, 2012 #1
    I had read that, according to Law of Buoyancy/Archimedes Principle,
    "Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object."

    But does that depend on the shape of the object immersed? I see, that a metal block doesn't float when it's immersed in water. Please help me understanding this.
     
  2. jcsd
  3. Jul 24, 2012 #2

    Doc Al

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    Staff: Mentor

    No, just on the volume of fluid displaced.
    Whether an object floats or not depends on how the buoyant force compares to the weight of the object. If the buoyant force is enough to counter the weight of the object, then it will float.

    If you submerge two identically shaped and sized blocks under water, the upward buoyant force on each is the same even though one might be wood and the other metal.
     
  4. Jul 24, 2012 #3
    It's compared to it's density not it's weight.
     
  5. Jul 24, 2012 #4

    Doc Al

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    No, the buoyant force must be compared to the weight of the object.

    You could express the condition for floating in terms of density: If the density of the object is less than the density of the fluid, then it will float. Which is a perfectly fine way to look at it. (Equivalent, of course.)
     
  6. Jul 24, 2012 #5

    russ_watters

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    Using density can be confusing for objects of nonuniform density, like ships.
     
  7. Jul 24, 2012 #6
    What's confusing about it if you just consider the overall density.Weight is where I get confused a ship weighing 10 ton floats a ten ton block of steel sinks.
     
  8. Jul 24, 2012 #7
    Yes, which was what I was trying to say but without explaining the bouyant force.If the overall density of the object is less than the fluid it is submerged in then the object floats if it's more it sinks.
    If the object floats the amount of fluid it displaces is equivalent to it's own weight.
     
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