Fluid Mechanics Physics

Buoyancy and Floatation

Basic Buoyancy and Floatation Concepts and Formulas




Buoyancy

When a body is immersed in a fluid, an upward force is exerted by the fluid on the body which is equal to the weight of the fluid displaced by the body. This upward force is called buoyant force and the phenomenon is called buoyancy.

Archimedes Principle

  • When a body is submerged either fully or partially then it is acted upon by a Force of Buoyancy in the vertical direction which is equal to the weight of liquid displaced by the body.
  • This Buoyant Force always acts through the centroid of liquid displaced.
  • Center of Buoyancy is that point through which buoyant force acts.

Principle of Flotation

  • According to this principle, if weight body is equal to the buoyant force then, the body will float.

Body = hρfluid

H = Height of Body

h = height of body submerged in fluid

Centre of Buoyancy

  • The point at which force of buoyancy acts is called centre of buoyancy.
  • It lies at the centre of gravity (G) of the volume of water displaced.

Metacenter

If a body which is floating in a liquid is given small angular displacement, it starts oscillating about some point ‘M’. This point is called Metacenter.

Metacentric Height (GM or MG) 

It is the distance between gravity centre and metacentre.

Condition for Equilibrium for Floating/Submerged Body

For Stable Equilibrium

  • In the case of a floating body, metacentre should be above the centre of gravity.
  • In the case of a submerged body, centre of buoyancy (B) should be above the centre of gravity (G).

For Unstable Equilibrium

  • In the case of a floating body, Metacenter (M) lies below Center of Gravity (G).
  • In the case of a submerged body, Center of Buoyancy (B) lies below Center of Gravity (G).

For Neutral Equilibrium

  • In the case of a floating body, M and G both coincides.
  • In the case of a submerged body, ‘B’ and ‘G’ both coincides.

Distance between metacentre and centre of buoyancy

BM = Imin/Vimmersed

Where, Imin = Moment of Inertia of top view of floating body about longitudinal axis

V = Volume of body immersed in liquid

Relation between B, G and M is given by

GM = I/V – BG

BG = Distance between CG of the whole body and CG of submerged part.

Also, If

  • GM > 0 (Stable Equilibrium)
  • GM < 0 (Unstable Equilibrium)
  • GM = 0 (Neutral Equilibrium)

Remember-

  • Metacenter height for rolling condition will be less than metacentric height for pitching condition.

Time Period of Oscillation

  • If a floating body oscillates then it’s time period of transverse oscillation is given by-

T = 2π sqrt(K2G/g×GM)

GM = Metacentric height

KG = Least Radius of Gyration



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