# Fluid Mechanics Questions and Answers – Pressure at a Point in Compressible Fluid

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This set of Fluid Mechanics Multiple Choice Questions & Answers (MCQs) focuses on “Pressure at a Point in Compressible Fluid”.

1. If the atmospheric pressure at sea level is 7.5 N/cm2, determine the pressure at a height of 3000m assuming the pressure variation follows isothermal law. The density of air is given as 1.2 km/m3.
a) 4.68 N/cm2
b) 9.37 N/cm2
c) 2.34 N/cm2
d) None of the mentioned

Explanation: pressure at any height Z = p*e-gZ/RT
=75000*e -9.81*3000*1.2/75000
= 4.68 N/cm2.

2. The barometric pressure at sea level is 760 mm of Mercury while that on a mountain top is 715 mm. If the density of air is assumed constant at 1.2 kg/m3 , what is the elevation of the mountain top?
a) 510 m
b) 1020 m
c) 255 m
d) 128 m

Explanation: Gauge pressure at any height h = pressure at sea level – pressure at that height
h=(9.81*13600*0.76)-9.81*13600*0.715)/1.2*9.81
=510 m.

3. Calculate the pressure at a height of 6500m above the sea level if the atmospheric pressure is 10.145 N/cm2 and temperature is 25℃ assuming air is incompressible. Take density of air as 1.2 kg/m3. Neglect variation of g.
a) 4.98 N/cm2
b) 2.49 N/cm2
c) 1.24 N/cm2
d) None of the mentioned

Explanation: Pressure= p – density of air*g*height
=101450-9.81*1.2*6500
= 2.49 N/cm2.
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4. Calculate the pressure of air at a height of 3500m from sea level where pressure and temperature of air are 10 N/cm2 and 25℃ respectively. The temperature lapse rate is given as 0.0065 ℃ /m. Take density of air at sea level equal to 1.2 kg/m3.
a) 19.7 N/cm2
b) 9.85 N/cm2
c) 4.93 N/cm2
d) 6.24 N/cm2

Explanation: pressure=p * (1-(k-1/k)*g*h*density/p)k/(k-1)
=9.85 N/cm2

Here, Lapse rate= -g/R*(k/k-1).

5. Pressure variation for compressible fluid is maximum for which kind of process?
a) Isothermal
c) Quasi Static
d) None of the mentioned

Explanation: Due to constant temperature, pressure variation for compressible fluid is maximum for isothermal process.

6. Why can’t the density be assumed as constant for compressible fluids?
a) It shows variation with temperature and pressure
b) It remains constant with temperature and pressure
c) It becomes almost constant at very high temperature
d) None of the mentioned

Explanation: Volume and hence density changes with change in temperature and pressure.

7.What is the variation observed in temperature in atmosphere with respect to elevation?
a) It goes on decreasing with height
b) It goes on increasing with height
c) It first increases then decreases
d) It first decreases then increases

Explanation: It goes on decreasing first and shows increase after 32000 m.

8. As we go upwards, at height there is slight decrease in pressure variation.
a) True
b) False

Explanation: There is slight decrease in pressure as value of g (acceleration due to gravity) decreases slightly as we go higher.

9. For dynamic fluid motion in a pipe, the pressure measurement cannot be carried out accurately by manometer.
a) True
b) False

Explanation: For fluid moving with variable velocity, fluctuation in pressure is frequent and more in magnitude. Hence, we cannot use manometer.

10. A simple U tube manometer connected to a pipe in which liquid is flowing with uniform speed will give which kind of pressure?
a) Absolute Pressure
b) Vacuum Pressure
c) Gauge Pressure
d) None of the mentioned

Explanation: A simple U tube manometer will give pressure with respect to atmosphere. Hence, it is gauge pressure.

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