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Bouyancy Mystifies Me

Discussion in 'Basic Scuba Discussions' started by pauldw, Oct 19, 2019.

  1. pauldw

    pauldw Solo Diver

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    Heavier part is going to roll toward the bottom, right?
     
  2. BlueTrin

    BlueTrin DIR Practitioner

    # of Dives: 50 - 99
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    If the cube is made of the same material, there is no ‘heavier/denser’ part.

    I think it is more about which position is more stable. In theory I think there are many positions which would be in equilibrium but in practice I think face up is more likely and less prone to be disturbed.
     
  3. tarponchik

    tarponchik Loggerhead Turtle

    # of Dives: 500 - 999
    Location: USA
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    Uniform
     
  4. tarponchik

    tarponchik Loggerhead Turtle

    # of Dives: 500 - 999
    Location: USA
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    I gave you a hint: figure out why a log of uniform density floats horizonally.
     
  5. Jcp2

    Jcp2 ScubaBoard Supporter ScubaBoard Supporter

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    The center of gravity of the uniform cube is in the center of the cube. The center of gravity of the volume of displaced water (in the shape of a truncated cube, or square box shorter than the wooden cube) is in the center of that box. That point is directly under the center of gravity of the cube. So the face of the cube is the most stable position for the cube to float in.
     
  6. FreeFlyFreak

    FreeFlyFreak Solo Diver

    # of Dives: 25 - 49
    Location: California
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    ZZZZZZZZZZ
     
    dead dog, happy-diver and Jcp2 like this.
  7. Jcp2

    Jcp2 ScubaBoard Supporter ScubaBoard Supporter

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  8. dberry

    dberry Hydrophilic ScubaBoard Supporter

    # of Dives: 100 - 199
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    Definitely a matter of stability. I suspect a log could be made to float vertically, on a perfectly calm day, etc. Just like you could carefully balance the log on end on flat ground. But it would be very unstable with respect to any slight horizontal force and would flip over to horizontal (a position of lower potential energy).
     
  9. tarponchik

    tarponchik Loggerhead Turtle

    # of Dives: 500 - 999
    Location: USA
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    The center of gravity of the displaced water is ALWAYS directly under the center of gravity of the cube, so this argument is mute. But you are thinking in the right direction, keep going!
     
  10. tarponchik

    tarponchik Loggerhead Turtle

    # of Dives: 500 - 999
    Location: USA
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    I read the 1st reference, they are mostly wrong. First, their statement that "The centre of buoyancy is the centre point of the mass below the water and is the point through which the buoyant force acts" is wrong. The centre of buoyancy is the centre point of the volume below the water, because it is the centre point of the displaced water. This makes no difference for uniform objects but for non-uniform objects, like boats and buoys, the difference is significant. Second, for any uniform object, the centre of buoyancy can not be located above the object's mass centre, so their figures are wrong and most of their arguments are wrong.
     

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