BUOYANCY AND STABILITY

 

TERMINOLOGY

Floating, Immersed body,Buoyant force

 

Archimedes’ Principle: body immersed in a fluid experiences a buoyant force equal to weight of fluid it displaces.

 

B

 
Which body is heavier? A or B

 

 

 

 

 

 

 


Which fluid is denser? A or B

 

 

 

 

 

 

 

 

 


Buoyant force FB

 

FB = weight of displaced fluid

 

       = g x (submerged volume of body)

 

 

HYDROMETER:

 

Instrument used to measure S.G of a fluid is based on principle of buoyancy

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


STABILITY

 

What is stability? 

A body subjected to small angular displacement will try to get back to original.

 

Or, moments that try to rotate body get it back to where it was before.

 

 

·      Two moments try to restore body.

·      FBxa and Wxa

·      Lesser of the two is called “Righting Moment”

Righting moment = Wxa  , if W<FB

                     =  FB xa, if W>FB

CRITERION FOR STABILITY OF A FULLY SUBMERGED BODY IS THAT CENTER OF BUOYANCY MUST BE ABOVE CENTER OF GRAVITY.

FLOATING BODY: W<FB

·      Body will rise until W=FB

·      At equilibrium, W and FB are equal and opposite and lie in the same vertical line

·      Recall, for a fully submerged body center of buoyancy (B)and center of gravity (G)are fixed and the criterion for stability is that B is above G

·      For a floating body, B can move relative to G, depending on how much fluid is displaced.



 

 

 




 

·      Stability of floating body depends on its shape.