# Viscosity Questions & Answers :: Fluid Mechanics

1). Water flows between two plates of which the upper one is stationary and the lower one is moving with a velocity V. What will be the velocity of the fluid in contact with the upper plate?

a) V
b) N ⁄ 2
c) 2V
d) 0

Explanation: According to the No-Slip condition, the relative velocity between the plate and the fluid in contact with it must be zero. Thus, the velocity of the fluid in contact with the upper plate is 0 and that with the lower plate is V.

2). The viscous force the relative motion between the adjacent layers of a fluid in motion.
Which one of the flowing fits best in the sentence?

a) opposes
b) never affects
c) facilitates
d) may effect under certain conditions

Explanation: Viscosity is the internal friction of a fluid in motion. It is the property by the virtue of which the relative motion between two adjacent fluid layers is opposed.

3). The viscosity of a fluid in motion is 1 Poise. What will be it’s viscosity (in Poise) when the fluid is at rest?

a) 0
b) 0.5
c) 1
d) 2

Explanation: Viscosity is the property of a fluid and is constant for a given fluid under given conditions, irrespective of the fact whether the fluid is at rest or in motion.

4). Which of the following correctly states how the viscosities of a liquid and a gas will change with temperature?

a) Viscosity increases with the increase in temperature of a liquid and decreases with the increase in temperature of a gas
b) Viscosity increases with the increase in temperature of a liquid and increases with the increase in temperature of a gas
c) Viscosity decreases with the increase in temperature of a liquid and decreases with the increase in temperature of a gas
d) Viscosity decreases with the increase in temperature of a liquid and increases with the increase in temperature of a gas

Explanation: The viscosity of a liquid is due to the cohesion between it’s molecules. With the increase in temperature of a liquid, cohesion increases, leading to the rise in viscosity. Viscosity of a gas is due to the momentum transfer between it’s molecules. With the increase in the temperature of a liquid, molecular motion increases, leading to a fall in viscosity.

5). Which one of the following is not a unit of dynamic viscosity?

a) Pa-s
b) N-s/m2
c) Poise
d) Stokes

Explanation:

SI unit of μ is N-s/m2 = Pa-s and CGS unit of μ is dyne-s/cm2. 1 Poise= 1 dyne-s/cm2 and 1 Stokes= 1 cm2/s. Thus, Stokes is not a unit of μ, rather it is a unit of kinematic viscosity υ.

6). Which of the following is a unit of dynamic viscosity?

a) [M1L1T-1].
b) [M1 L-1 T-1].
c) [M1 L-2 T-2].
d) [M1 L-2 T-2].

7). Which one of the following is the CGS unit of dynamic viscosity?

a) Stokes
b) Pa-s
c) m2/s
d) Poise

Explanation:  CGS unit of μ is = dyne-s/cm2. 1 Poise= 1 dyne-s/cm2 and 1 Stokes= 1 cm2/s. Thus, the CGS unit of μ will be Poise. Stokes is the CGS unit of kinematic viscosity.

8). The dynamic viscosity of a fluid is 1 Poise. What should one multiply to it to get the answer in N-s/m2?

a) 0.1
b) 1
c) 10
d) 100

9). Which of the following is a unit of kinematic viscosity?

a) Stokes
b) Pa-s
c) m2=s
d) Poise

Explanation:

Thus, the unit of ν is cm2/s = Stokes Poise is the unit of dynamic viscosity.
1 Poise = 1 dyne-s/cm2

10). Which of the following is the dimension of kinematic viscosity?

a) [L1T-1].
b) [L1T-2].
c) [L2 T-1].
d) [L2 T-2].

11). The kinematic viscosity of a fluid is 0.1 Stokes. What will be the value is m2/s?

a) 10-2
b) 10-3
c) 10-4
d) 10-5

Explanation: 1Stokes = 1cm2/s = 10-4m2/s Therefore, 0.1Stokes = 10-1cm2/s = 10-5m2/s.

12). The shear stress at a point in a liquid is found to be 0.03 N/m2. The velocity gradient at the point is 0.15 s-1. What will be it’s viscosity (in Poise)?

a) 20
b) 2
c) 0.2
d) 0.5

13). The space between two plates (20cm*20cm*1cm), 1 cm apart, is filled with a liquid of viscosity 1 Poise. The upper plate is dragged to the right with a force of 5N keeping the lower plate stationary.

What will be the velocity in m/s of flow at a point 0.5 cm below the lower surface of the upper plate if linear velocity profile is assumed for the flow?

a) 1.25
b) 2.5
c) 12.5
d) 0.25

14. Two horizontal plates placed 250mm have oil of viscosity 20 poises. Calculate the shear stress in oil if the upper plate is moved with velocity of 1250mm/s.
a) 20 N/m2
b) 2 N/m2
c) 10 N/m2
d) None of the mentioned

Explanation: Shear Stress = Viscosity * Velocity Gradient
= 20/10 × 1.25/0.25
= 10 N/m2.

15. The kinematic viscosity of oil of specific gravity .8 is .0005 .This oil is used for lubrication of shaft of diameter .4 m and rotates at 190 rpm. Calculate the power lost in the bearing for a sleeve length of 90mm. The thickness of the oil film is 1.5mm.

a) 477.65 Watts
b) 955.31 Watts
c) 238.83 Watts
d) None of the mentioned

Explanation:

Power lost = torque × angular velocity
= force × radius × angular velocity

= shear stress × area × radius × angular velocity

Shear Stress = viscosity × velocity gradient

Power lost = 0.0005 × 0.8 × 1000 × 2 × 3.142 × 190/60 × 0.2 × 3.142 × 0.23 × 190/60
= 477.65 Watts.

16. Find the kinematic viscosity of oil having density 1962 g/m3. the force experienced for area of 20 m2 is 4.904 kN and velocity of gradient at that point is 0.2/s.

a) 0.625
b) 1.25
c) 2.5
d) None of the mentioned

Explanation: kinematic viscosity = dynamic viscosity / density
= (shear stress × density)/velocity gradient
= (4904 × 1962)/(20 × 0.2)
= .625.

17. The velocity distribution for fluid flow over a flat plate is given by u=2y-6y2 in which u is the velocity in metre per second at a distance of y metre above the plate. Determine the shear stress at y=0.15m.Take dynamic viscosity of fluid as 8.6 poise.

a) 0.172 N/m2
b) 0.344 N/m2
c) 0.086 N/m2
d) None of the mentioned

Explanation: for y = 0.15m, velocity gradient = 0.2

shear stress = 0.86 × 0.2 = 0.172N/m2.

18. In which types of fluids it is observed that momentum transfer dominates cohesive forces with increase in temperature and hence viscosity increases
a) Gases
b) Liquids
c) Solids
d) None of the mentioned

Explanation: It is the characteristic property of gases that show an increase in viscosity with an increase in temperature.

19. What is the characteristic variation shown by the thixotropic fluids in their shear stress vs. rate of shear strain graph?

a) shear stress increases with increase in rate of shear strain
b) shear stress decreases with increase in rate of shear strain
c) shear stress shows variation only after a definite shear stress is reached
d) shear stress has decreasing constant and then variation relationship with rate of shear strain

Explanation: Thixotropic fluid show a Non-Newtonian variation for shear stress vs. rate of shear strain graph after a characteristic limiting value of shear stress is reached.

20. What happens to viscosity in the case of incompressible fluids as temperature is increased?

a) It remains constant
b) It increases
c) It decreases
d) None of the mentioned

Explanation: In case of incompressible fluids, cohesive forces govern the viscosity. As temperature increases the cohesive forces between fluid molecules decreases due to increase in molecular agitation. Hence, as a result, viscosity decreases.

21. If a fluid, which has a constant specific gravity, is taken to a planet where acceleration due to gravity is 3 times compared to its value on earth, what will happen to its kinematic viscosity.

a) It increases
b) It decreases
c) It remains constant
d) None of the above

Explanation: Kinematic viscosity depends on density and dynamic viscosity. Both, density and dynamic viscosity, are independent of acceleration due to gravity. Therefore, kinematic viscosity is independent of acceleration due to gravity.

22. In liquids in order to measure the viscosity of fluid experimentally we consider the variation of shear stress with respect to what property?

a) strain
b) shear strain
c) rate of shear strain
d) none of the mentioned