Hydrostatics: Buoyancy & Stability

Buoyancy

Archimedes' Principle: A body immersed in a fluid experiences a vertical upward buoyant force equal to the weight of the fluid it displaces.

FB=γVdisplacedF_B = \gamma V_{displaced}

  • Floating Body: Weight of body = Buoyant Force (W=FBW = F_B)
  • Submerged Body: Apparent Weight = True Weight - Buoyant Force

Stability

Stability refers to the ability of a body to return to its original position after a small disturbance (tilt).

Submerged Bodies

  • Stable: Center of Gravity (GG) is below Center of Buoyancy (BB).
  • Unstable: GG is above BB.

Floating Bodies

Stability is determined by the Metacenter (MM). The Metacenter is the intersection of the buoyant force vector before and after rotation.

Metacentric Height (GM)

Stable Equilibrium: MM is above GG (GM>0GM > 0). Unstable Equilibrium: MM is below GG (GM<0GM < 0).

MG=MB±GBMG = MB \pm GB

Distance from Buoyancy Center to Metacenter (MBMB): MB=IVdispMB = \frac{I}{V_{disp}}

  • II = Moment of inertia of waterline area about tilt axis.
  • VdispV_{disp} = Volume of displaced fluid.

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