Fluid Mechanics Questions and Answers Part-6

1. The value of the Bulk Modulus of elasticity for an incompressible fluid is
a) zero
b) unity
c) infinity
d) very low

Answer: c
Explanation: k = 1/β, where k= Bulk Modulus of elasticity and β= compressibility. For an incompressible fluid, β=0, thus the value of k will tend to infinity.

2. Three fluids 1, 2 and 3 have Bulk Moduli of k1, k2 and k3 respectively. If k1 > k2 > k3, which liquid will have the highest compressibility?
a) liquid 1
b) liquid 2
c) liquid 3
d) they’ll have equal compressibilities

Answer: c
Explanation: k = 1=β, where k= Bulk Modulus of elasticity and β= compressibility. If k1 > k2 > k3, then β1 < β2 < β3. Thus, liquid 3 will have the highest compressibility.

3. Bulk Modulus, Pressure, Force, Stress – Which one of these won’t have the same unit as the others?
a) Bulk Modulus
b) Pressure
c) Force
d) Stress

Answer: c
Explanation: The SI unit of Bulk Modulus, Pressure and Stress is N/m2 but the unit of Force is N.

4. Which of the following is the dimension of Bulk Modulus?
a) [M1L-1T-1].
b) [M1L-1T-2].
c) [M1L1T-2].
d) [M1L1T-1].

Answer: b
Explanation:
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5. Which one of the following is the unit of compressibility?
a) m/N
b) m2/N
c) m3/N
d) it is unitless

Answer: b
Explanation: k = 1/β, where k= Bulk Modulus of elasticity and β= compressibility.
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Thus the unit of Bulk modulus is N/m2 and the unit of compressibility becomes m2/N.

6. Which of the following is the dimension of compressibility?
a) [M1L1T-2].
b) [M1L1T-1].
c) [M-1L1T-2].
d) [M-1L1T2].

Answer: d
Explanation: k = 1/β, where k = Bulk Modulus of elasticity and β= compressibility.
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and [β] = [1/k] = [M-1L1T2]

7. Which of the following contribute to the reason behind the origin of surface tension?
a) only cohesive forces
b) only adhesive forces
c) neither cohesive forces nor adhesive forces
d) both cohesive forces and adhesive forces

Answer: d
Explanation: The molecules on the surface of a liquid experience cohesive forces due to surrounding liquid molecules acting downward and adhesive forces due to surrounding gaseous molecules acting upwards. Surface tension orginates due to this unbalanced force on the surface molecules.

8. What will be the diameter (in mm) of a water droplet, the pressure inside which is 0.05 N/cm2 greater than the outside pressure? (Take surface tension as 0.075 N/m)
a) 3
b) 0.3
c) 0.6
d) 6

Answer: c
Explanation: p = 4σ/d
where p = pressure difference between the liquid droplet and the surrounding medium, σ = surface tension and d = diameter of the droplet. Substituting all the values,
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9. A soap bubble of d mm diameter is observed inside a bucket of water. If the pressure inside the bubble is 0.075 N/cm2, what will be the value of d? (Take surface tension as 0.075 N/m)
a) 0.4
b) 0.8
c) 1.6
d) 4

Answer: b
Explanation: p = 8σ/d
where p = pressure difference between the bubble and the surrounding medium, σ = surface tension and d = diameter of the bubble. Substituting all the values,
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10. A liquid jet of 5 cm diameter has a pressure difference of N/m2. (Take surface tension as 0.075 N/m)
a) 12
b) 6
c) 3
d) 1.5

Answer: d
Explanation: p = σ/d
where p = pressure difference between the bubble and the surrounding medium, σ = surface tension and d = diameter of the bubble. Substituting all the values,
p = 0.075 / 5 * 10-2 = 1.5 N/m2.