Engineering Physics Questions and Answers – Significance of Rotational Motion

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

1. A flywheel has a mass of 25kg has a radius of 0.2m. What force should be applied tangentially to the rim of the flywheel so that it acquires an angular acceleration of 2rad/s2?
a) 2N
b) 5N
c) 3N
d) 1N

Explanation: M = 25kg, R = 0.2m, α = 2rad/s2
Moment of inertia of the flywheel about its axis,
I = 1/2 MR2=1/2×25×0.22=0.5kgm2
As torque, τ = F×R=I×α
Therefore, Force F = Iα/R=(0.5×2)/0.2=5N.

2. A torque of 10Nm is applied to a flywheel of mass 1kg and radius of gyration 50cm. What is the resulting angular acceleration?

Explanation: τ=10Nm, M = 10kg, K = 0.50m
As τ=I×α = MK2 α

3. The angular momentum of a body is 3.14 is and its rate of revolution is 10 cycles per second. Calculate the moment of inertia of the body about the axis of rotation.
a) 5 kgm2
b) 0
c) 31.4 kgm2
d) 0.5 kgm2

Explanation: L = 3.14 Js v = 10 rps
As L = Iω
I = L/ω = 3.14/(2×3.14×10)=0.5kgm2.

4. The angular velocity of a planet revolving in an elliptical orbit around the sun increases, when it comes closer to the sun. This is due to ______________
a) Conservation of momentum
b) Newton’s second law of motion
c) Conservation of angular momentum
d) Law of angular velocity

Explanation: The angular velocity of a planet revolving in an elliptic orbit around the sun increases when it comes closer to the sun increases when it comes closer to the sun because its moment of inertia about the axis through the sun decreases. When it goes far away from the sun, its moment of inertia decreases so as to conserve angular momentum.

5. An ice skater or a ballet dancer can decrease her angular velocity by folding her arms and bringing the stretched leg close to the other leg.
a) True
b) False

Explanation: An ice skater can increase her angular velocity by folding her arm and bringing the leg closer to the other. When she stretches her hand outward, her moment of inertia increases and hence angular speed decreases to conserve angular momentum. When she folded her hands and brings the stretched legs close to the other leg, her moment of inertia decreases and hence her angular speed increases.
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6. The centre of mass of a body should lie inside the body.
a) True
b) False

Explanation: The centre of mass of a body need not necessarily lie inside the body. The centre of mass of a ring lies in its hollow portion.

7. In a hand driven grinding machine, the handle is put _____________
a) Perpendicular to the stone
b) Near the circumference of the stone
c) A few distances above the stone
d) As far away from the stone

Explanation: For a given force, torque can be increased if the perpendicular distance of the point of application of the force from the axis of rotation is increased. Hence the handle put near the circumference produces maximum torque.

8. A faulty balance with unequal arms has its beam horizontal. The weights of the two pans are _____________
a) Equal
b) Unequal
c) Uncertain
d) Very large

Explanation: They are of unequal mass. Their masses are in the inverse ratio of the arms of the balance.

9. Which part of the ship is made heavy?
a) The whole ship should be light in weight
b) The top part
c) The bottom part
d) The sides

Explanation: The bottom of the ship is made heavy so that the centre of gravity remains low. This ensures the stability of its equilibrium.

10. Standing is not allowed in double decker bus.
a) True
b) False

Explanation: When the passengers stand in the upper deck, the centre of gravity of the loaded bus is raised this makes it less stable.

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