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Mechanical Properties Of Fluids

Question
CBSEENPH11018246

State and prove Archimedes principle.

Solution

Archimedes principle: When a body is immersed partially or wholly in a liquid, its weight appears to be reduced and loss of weight is equal to the weight of liquid displaced by the body.
                  

Proof: Consider a body of height h and area of cross-section A placed in liquid of density σ at a depth x below the free surface.
Let ρ be density of the body.

Now the pressure at the upper face of the body is,
                     straight P subscript 1 equals straight P subscript 0 plus xσg 
where,  Pis atmospheric pressure.
Pressure at the lower face of the body is, 
                  straight P subscript 2 equals straight P subscript 0 plus left parenthesis straight h plus straight x right parenthesis σg
Therefore downward thrust on  upper face of the body is,
                  straight F subscript 1 equals straight P subscript 1 straight A
and upward thrust on lower face of body is,
                  straight F subscript 2 equals straight P subscript 2 straight A
Since P> P1  therefore F> F

Body will experience the net force F- Fin upward direction known as upward thrust.

therefore space space space space space straight U equals straight F subscript 2 minus straight F subscript 1 equals left parenthesis straight P subscript 2 minus straight P subscript 1 right parenthesis straight A space

space space space space space space space space space space space equals straight A open square brackets open parentheses straight h plus straight x close parentheses σg minus xσg close square brackets equals Ahσg
italic space italic space italic space italic space italic space italic space italic space italic space italic space italic space
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where, V is volume of body
        = Weight of liquid displaced by body.

Now, the  body experiences two forces: weight of the body itself and upward thrust.
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i.e. Apparent weight is less than the actual weight by amount equal to weight of liquid displaced by body.