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Making sure my intuition is correct about buoyant force

  1. Apr 18, 2012 #1
    Hi I want to make sure my intuition is correct about the buoyant force.
    What I am confused by if we take a cube or some sort of object and its submerged at height H1 as in picture I uploaded,then their will be a force of P1 * A; in case of the cube it's it would be d^2. What I don't understand or I am not sure that my intuition is correct is this is why the height of H2(height 2 not Hydrogen gas :D) is taken from the top of the water to bottom of the cube instead of bottom of the water to the bottom of the cube as picture 2 represent.

    My intuition to this is as follows since the upward pressure P2 is same throughout the liquid then P2 if taken from frame of reference from above of the beaker it would be the same that if taken from frame of reference from bottom of the beaker as picture 2 represent.Is that intuition is correct ?


    Please look at picture as they explain better than I do.
    Thanks in advance.
     

    Attached Files:

  2. jcsd
  3. Apr 18, 2012 #2

    Doc Al

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    Staff: Mentor

    What matters is the fluid pressure at any point. That pressure depends on the depth below the surface, not the height above some bottom point.
     
  4. Apr 18, 2012 #3
    Second picture is correct. So, for the pressure in point 1 you will take h1, and for the pressure in pont 2 you will take (h1+h2). O course, pressure in the point 2 will be greater than pressure in the point 1, and you will have resultant force upward. Now you have to calculate weight and you will see will the body be stable, go up, go down, or whatever..
     
  5. Apr 18, 2012 #4

    Doc Al

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    Staff: Mentor

    The key point is that the pressure on the cube will be the same regardless of its distance from the bottom as long as its depth below the surface is the same. The height above the bottom of the container has no bearing on the fluid pressure.
     
  6. Apr 18, 2012 #5
    I see what made me think this way about picture 1 is because is an upward force so I didn't visualize it correctly because I thought if its taken from bottom then it would make the resultant force upward not downward thats what made me confused.
     
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