# Engineering Physics Questions and Answers – Thrust of a Liquid

This set of Engineering Physics Questions and Answers for Campus interviews focuses on “Thrust of a Liquid”.

1. What will be the length of a mercury column in a barometer tube, when the atmospheric pressure is 75cm of mercury and the tube is inclined at an angle of 60° with the horizontal direction?
a) 8.66cm
b) 86.6cm
c) 866cm
d) 0.866cm

Explanation: If l is the length of a mercury column in the barometer tube, then
h/l=sin60
75/l=√3/2
l=(75×2)/√3=86.6cm.

2. The density of the atmosphere at sea level is 1.29kgm-3. Assume that is does not change with altitude. Then how high would the atmosphere extend? (g=9.8mm-2)
a) 0.5km
b) 0.8km
c) 8km
d) 80km

Explanation: Pa=hρg
h=Pa/ρg=(1.01×105)/(1.29×9.81)=7981m=8km(approx).

3. It is painful to walk barefooted on the ground covered with edged pebbles.
a) True
b) False

Explanation: While walking, when the entire weight of our body gets supported on the sharp edge of any pebble, it will exert a large pressure on our feet due to the reaction. This causes considerable pain on our feet.

4. A closed compartment containing gas in moving with some acceleration on horizontal direction. Neglect effect of gravity. Then the pressure in the compartment is ___________
a) Same everywhere
b) Lower in the front side
c) Lower in the rear side
d) Lower in the upper side

Explanation: As the compartment has acceleration in the forward direction, the gas molecules experience an acceleration in the backward direction. This lowers the pressure in the compartment in the front side.

5. Statement 1: The stream of water flowing at high speed from a garden hose pipe tends to spread like a fountain when held vertically up, but tends to narrow down when held vertically down.
Statement 2: In any steady flow of an incompressible fluid, the volume flow rate of the fluid remains constant.
a) Statement 1 is true, statement 2 is true. Statement 2 is a correct explanation for statement 1
b) Statement 1 is true, statement 2 is true. Statement 2 is not a correct explanation for statement 1
c) Statement 1 is true, statement 2 is false
d) Statement 1 is false, statement 2 is true

Explanation: Volume rate of flow, Q=Av = constant
As speed of upstream decreases, its area of cross-section increases. As the speed of downstream increases, its area of cross-section decreases.

6. If two soap bubbles of different radii are connected by a tube ___________
a) Air flow from the bigger bubble to the smaller bubble till the sizes become equal
b) Air flows from bigger bubble to the smaller bubble till the sizes are interchanged
c) Air flows from the smaller bubble to the bigger
d) There is no flow of air

Explanation: The excess pressure inside of a soap bubble is inversely proportional to its radius. So, the pressure is more inside the smaller bubble than the bigger bubble. When these two bubbles are connected by a tube, air will flow from the smaller bubble to the bigger bubble.

7. A 20cm long capillary tube is dipped in water. The water rises up to 8cm. If the entire arrangement is put in a freely falling elevator the length of the water column in the capillary tube will be ___________
a) 4cm
b) 20cm
c) 8cm
d) 10cm

Explanation: In the freely falling elevator, the entire arrangement is in a state of weightlessness, g = 0. So, water will rise (h = 2σcosθ/rρg ) to fill the entire 20cm length of the tube.

8. Assertion: A this stainless steel needle can lay floating on a still water surface.
Reason: Any objection floats, when the buoyancy force balances the weight of the object.
a) Both assertion and reason are true and the reason is the correct explanation of the assertion
b) Both assertion and reason are true but the reason is not the correct explanation of the assertion
c) Assertion is true but the reason is false
d) Assertion and reason is false

Explanation: The assertion is true but the reason is false. The needle floats when the upward tension on the needle balances its weight.

9. At a critical temperature, the surface tension of a liquid ___________
a) Is zero
b) Is infinity
c) Is the same as any other temperature
d) Cannot be determined

Explanation: At a critical temperature, the surface tension of a liquid becomes zero.

10. In a capillary tube experiment, a vertical, 30 cm long capillary tube is dipped in water. The water rises up to a height of 10 cm due to capillary action. If this experiment is conducted in a freely falling elevator, the length of the water column becomes ___________
a) 10 cm
b) 20 cm
c) 30 cm
d) Zero

Explanation: In the freely falling elevator, g = 0.
Water will rise (h=2σcosθ/rρg) to fill the entire 30 cm length of the tube.

Sanfoundry Global Education & Learning Series – Engineering Physics.

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