Question

Water is flowing on a horizontal fixed surface, such that its flow velocity varies with $$y$$ (vertical direction) as $$v = k\left( {\frac{{2{y^2}}}{{{a^2}}} - \frac{{{y^3}}}{{{a^3}}}} \right).$$    If coefficient of viscosity for water is $$\eta ,$$ what will be shear stress between layers of water at $$y = a.$$

A. $$\frac{{\eta k}}{a}$$  
B. $$\frac{\eta }{{ka}}$$
C. $$\frac{{\eta a}}{k}$$
D. None of these
Answer :   $$\frac{{\eta k}}{a}$$
Solution :
Newton’s law of viscosity, $$F = \eta A\frac{{dv}}{{dy}}$$
Stress $$ = \frac{F}{A} = \eta \left( {\frac{{dv}}{{dy}}} \right) = \eta k\left( {\frac{{4y}}{{{a^2}}} - \frac{{3{y^2}}}{{{a^3}}}} \right)$$
At $$y = a,$$  stress $$ = \eta k\left( {\frac{4}{a} - \frac{3}{a}} \right) = \frac{{\eta k}}{a}$$

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Basic Physics >> Mechanical Properties of Solids and Fluids

Releted Question 1

The following four wires are made of the same material. Which of these will have the largest extension when the same tension is applied?

A. $$length=50\,cm, \,\,diameter=0.5 \,mm$$
B. $$length = 100 \,cm, \,\,diameter= 1 \,mm$$
C. $$length= 200 \,cm, \,\,diameter= 2 \,mm$$
D. $$length=300 \,cm, \,\,diameter =3 \,mm$$
Releted Question 2

A U-tube of uniform cross section (see figure) is partially filled with a liquid I. Another liquid II which does not mix with liquid I is poured into one side. It is found that the liquid levels of the two sides of the tube are the same, while the level of liquid I has risen by $$2 \,cm.$$  If the specific gravity of liquid I is $$1.1,$$  the specific gravity of liquid II must be-
Mechanical Properties of Solids and Fluids mcq question image

A. $$1.12$$
B. $$1.1$$
C. $$1.05$$
D. $$1.0$$
Releted Question 3

A homogeneous solid cylinder of length $$L\left( {L < \frac{H}{2}} \right),$$   cross-sectional area $$\frac{A}{5}$$ is immersed such that it floats with its axis vertical at the liquid-liquid interface with length $$\frac{L}{4}$$ in the denser liquid as shown in the figure. The lower density liquid is open to atmosphere having pressure $${P_0}.$$ Then density $$D$$ of solid is given by-
Mechanical Properties of Solids and Fluids mcq question image

A. $$\frac{5}{4}d$$
B. $$\frac{4}{5}d$$
C. $$4d$$
D. $$\frac{d}{5}$$
Releted Question 4

A large open tank has two holes in the wall. One is a square hole of side $$L$$ at a depth $$y$$ from the top and the other is a circular hole of radius $$R$$ at a depth $$4y$$  from the top. When the tank is completely filled with water, the quantities of water flowing out per second from both holes are the same. Then, $$R$$ is equal to-

A. $$\frac{L}{{\sqrt {2\pi } }}$$
B. $$2\pi L$$
C. $$L$$
D. $$\frac{L}{{2\pi }}$$

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