# Engineering Physics Questions and Answers – Buoyancy

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This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Buoyancy”.

1. In incompressible fluid flows steadily through a cylindrical pipe which had a radius 2R at point A and R at a point B further along the flow direction. If the velocity at A is v, then that at B is?
a) v/2
b) v
c) 2v
d) 4v

Explanation: Using equation of continuity,
a1 v1=a2 v2
π(2R)2 v=πR2 v2
v2=4v.

2. When the temperature is increased, the angle of contact of a liquid?
a) Increase
b) Decrease
c) Remains the same
d) First increases and then decreases

Explanation: The angle of contact of liquid increases with the increase of temperature.

3. Assertion: A hydrogen filled balloon stops using after it has attained a certain height in the sky.
Reason: The atmospheric pressure decreases with height and becomes zero when maximum height is attained.
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. When the atmospheric pressure becomes equal to the pressure inside the balloon, the balloon stops rising.

4. A cube of wood floating in water supports a 200g mass at the centre of its top face. When the mass is removed, the mass rises by 2cm. Determine the volume of cube.
a) 1000cm3
b) 100cm3
c) 10cm3
d) 1cm3

Explanation: Let the side of the cube be l cm. As the mass 200g is removed, the cube rises by 2 cm. So by law of floatation,
Upthrust on cube due to displaced volume of water = 200gf
l×l×2×l×g=200×g
l2=100
So, l=10cm
Therefore, volume of the cube = 103=1000cm3.

5. A solid weight 6kg in air. If its density is 2000kgm-3, what will be its apparent weight in water?
a) 6kg
b) 2kg
c) 3kg
d) 5kg

Explanation: Volume of solid=mass/density=6/2000 m3=Volume of water displaced
Mass of water displaced=Volume×Density=6/2000×1000=3kg
Apparent weight in water = 6-3 = 3kg.

6. A boat having a length of 3m and breadth 2m is floating on a lake. The boat sinks by one cm when a man gets on it. What is the mass of the man?
a) 6kg
b) 60kg
c) 16kg
d) 600kg

Explanation: Fraction of volume above water surface = 1 – 0.90 = 0.10
Weight of man = Weight of water displaced by boat when the man gets in
m×9.8=(3×2×0.01)×103×9.8
Hence, m = 60kg.

7. It is difficult to cook food at the mountain.
a) True
b) False

Explanation: The pressure inside the pressure cooker is very high. This rises the boiling point of water and the temperature inside the cooker is higher than 100°C which results in faster cooking of food. At the mountains, the pressure is less, so the boiling point of water is less than 100°C. This makes the cooking of food difficult.

8. Spherical balls of radius R are falling in a viscous fluid of viscosity Ƞ with a velocity v. The retarding viscous force acting on the spherical ball is ___________
a) Directly proportional to R but inversely proportional to v
b) Directly proportional to both radius R and velocity v
c) Inversely proportional to both radius R and velocity v
d) Inversely proportional to R but directly proportional to velocity v

Explanation: F = 6πȠRv, which is retarding viscous force is directly proportional to both R and v.

9. A wooden block is taken to the bottom of a deep, calm lake of water and then released. It rises up with a ___________
a) Constant acceleration
b) Decreasing acceleration
c) Constant velocity
d) Decreasing velocity

Explanation: The wooden block rises up with acceleration,
a=(Upthrust-weight of block)/(Mass of block).

10. The water droplets in free fall are spherical due to ___________
a) Gravity
b) Viscosity
c) Surface tension
d) Intermolecular attraction

Explanation: Freely falling water droplets assume spherical shape due to the surface tension of water.

11. Assertion: A needle placed carefully on the surface of the water may float, whereas a ball of the same material will always sink.
Reason: The buoyancy of an object depends both on the material and shape 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 because the force of surface tension on it is enough to balance its weight. In the case of the ball, the resultant of the force of surface tension and buoyancy is enough to balance the weight of the ball.

Sanfoundry Global Education & Learning Series – Engineering Physics.

To practice all areas of Engineering Physics, here is complete set of 1000+ Multiple Choice Questions and Answers. 