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W is the work done in forming a bubble of radius r, the work is done in forming a bubble of radius 2r will be:-

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Explanation

1W=T×A=T×4πr2W  r2

Equation of continuity based on :

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Explanation

It is based on the conservation of mass

Bernoulli's theorem is based on :

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Explanation

Conceptual

Two spherical soap bubbles of radii r1 and r2 in vaccum collapse under isothermal condition. The resulting bubble has radius equal to :

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Explanation

4for isothermal conditionPV = P1V1 + P2V24Tr43πr3 = 4Tr143πr13 + 4Tr2.43πr23r2 = r12 + r22

The energy needed in breaking of a drop of liquid of radius R into n drops of radius r is given by (T is surface tension and P is atmospheric pressure) :

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Explanation

Surface area of large sphere=4πR2Surface area of n small spheres=n4πr2Work done=T×A=T×n4πr2-4πR2

 

A rectangular film of liquid is extended from (4 cm× 2 cm) to 5 cm×4×cm. If the work done is 3×10-4 J,the value of the surface tension of the liquid is 

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Explanation

 

(b) Increases in surface energy= work done in area×Surface tension 

Increase in surface area,

 A=5×4-4×2×2                       flim has two surfaces

=20-8×2 cm2=24 cm2=24×10-4 m2 

So, work done, W=T.A

  3×10-4=T×24×10-4 T=18=0.125 N/m

 

Three liquids of densities ρ1, ρ2 and  ρ3 (with p1>p2>p3), having the same value of surface tension T, rise to the same height in three identical capillaries. The angles of contact θ1,θ2 and θ3 obey

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Explanation

 

(b) According to ascent formula for capillary tube,

              h=2Tcos θpgr cosθ1p1=cosθ2p2=cosθ3p3Thus,cosθρ p1>p2>p3 cos θ1>cosθ2>cos θ3   0θ1<θ2<θ3<π2

 

Two non-mixing liquids of densities ρ and nρ(n>1) are put in a container. The height of each liquid is h. A solid cylinder  floats with its axis vertical and length pL p<1 in the denser liquid. The density of the cylinder is d. The cylinder floats with its axis vertical and length pL (p<1) in the denser liquid. The density d is equal to 

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Explanation

According to Archimedes' principle, the buoyant force on the cylinder is equal to the weight of the liquid displaced by the cylinder. The volume of the liquid displaced is pLπr^2, where r is the radius of the cylinder. The weight of this liquid is pLπr^2ρn. This weight must equal the weight of the cylinder, which is pLπr^2d. Therefore, d = ρn.

The approximate depth of an ocean is 2700 m. The compressibility of water is 45.4 x 10-11 Pa-1 and density of water is 103kg/m3. What fractional compression of water will be obtained at the bottom of the ocean?

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Explanation

Given d=2700 m

ρ=103kg/m3

Compressibility=45.4 x 10-11 Pa-1  per pascal

The pressure at the bottom of ocean is given by

P=ρgd=103x10x2700=27x106Pa

So, fractional compression=compressibility x pressure

=45.4 x 10-11x 27 x106

=1.2x10-2

A wind with speed 40 m/s blows parallel to the roof of a house. The area of the roof is 250 m2. Assuming that the pressure inside the house is atmospheric pressure, the force exerted by the wind on the roof and the direction of the force will be (Pair=1.2kg/m3)

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Explanation

From Bernoulli's theorem

p1+12ρv12=p2+12ρv22 

where, p1 and p2 are pressure inside and outside the roof and v1 v2 are velocities of wind inside and outside the roof, Neglect the width of the roof.

Pressure difference is

p1-p2=12ρ(v22-v12)

=12x1.2(402-0)=960N/m2

Force acting on the roof is given by

F=(p1-p2)A=960 x 250=24x104N=24X105N

As the pressure inside the roof is more than outside to it. So the force will act m the upward direction, i.e. F=24x105N,upward



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