# Engineering Physics Questions and Answers – Moment of Inertia and its Significance

This set of Engineering Physics Questions and Answers for Aptitude test focuses on “Moment of Inertia and its Significance”.

1. The instantaneous speed of the point of contact during rolling is zero.
a) True
b) False

Explanation: A rolling body can be imagined to be rolling about an axis passing through the point of contact of the body and the ground. Hence the instantaneous speed of the point of contact is zero.

2. A mass m is moving with a constant velocity along a line parallel to the x-axis, away from the origin. Its angular momentum with respect to the origin ___________
a) Is zero
b) Remains constant
c) Goes on increasing
d) Goes on decreasing

Explanation: Angular momentum = Mass of momentum
L = mv×h = constant
As the particle moves, m, v and h all remain unchanged.

3. A particles undergoes uniform circular motion. About which point on the plane of the circle, will the angular momentum of the particles remain conserved?
a) Centre of the circle
b) On the circumference of the circle
c) Inside the circle
d) Outside the circle

Explanation: In uniform circular motion, centripetal force acts towards the centre. Torque due to such a force about the centre is zero. Hence the angular momentum is conserved about the centre of the circle.

4. If the resultant of all the external forces acting on a system of particles is 0, then from an inertial frame, one can surely say that ___________
a) Linear momentum of the system does not change in time
b) Kinetic energy of the system does not change in time
c) Angular momentum of the system does not change in time
d) Potential energy of the system does not change in time

Explanation: According to the law of conservation of linear momentum if external force is 0 then the linear momentum of the system does not change in time. There may be external forces acting due to which kinetic energy or potential energy or both may change. Also, net force is 0 does not mean net torque is 0. So angular momentum may change. Hence only linear momentum of the system does not change in time.

5. Statement 1: If there is no external torque on the body about its centre of mass, then the velocity of the centre of mass remains constant.
Statement 2: The linear momentum of the isolated system remains constant.
a) Statements 1 and 2 are true and statement 2 is the correct explanation of statement 1
b) Statement one and two are true and statement2 is not the correct explanation of statement1
c) Statement 1 is true, statement 2 is false
d) Statement 2 is true, statement 1 is false

Explanation: The absence of external torque does not ensure the absence of external force. If an external force is present, then the velocity of the centre of mass will not remain constant. Thus statement 1 is false.
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6. If a person standing on a rotating disc stretches out his hands, the angular speed will ___________
a) Increase
b) Decrease
c) Remains same
d) Increases and then decreases

Explanation: As the person stretches out his hands, his moment of inertia increases. To conserve angular momentum, his angular speed increases.

7. Analogue of mass in rotational motion is ___________
a) Moment of inertia
b) Angular momentum
c) Gyration
d) Angular acceleration

Explanation: Analogue of mass in rotational motion is the angular momentum

8. Moment of inertia of an object does not depend upon ___________
a) Mass of object
b) Mass of distribution
c) Angular velocity
d) Axis of rotation

Explanation: Moment of inertia does not depend on the angular velocity of the object but depends on all factors given in other options.

9. Statement 1: If polar ice melts, days will be longer.
Statement 2: Moment of inertia decreases and thus angular velocity increases.
a) Both statement 1 and 2 are true and statement2 is the correct explanation of the statement1
b) Both statement 1 and 2 are true but the statement2 is not the correct explanation of the statement1
c) Statement 1 is true but statement 2 is false
d) Statement 1 and 2 are false

Explanation: As the polar ice melts, water so formed flows towards the equator. The moment of inertia of the earth increases. To conserve angular momentum, angular velocity decreases. This increases the length of the day.

10. The moment of inertia of a body about a given axis is 1.2kgm2. Initially, the body at rest. In order to produce a rotational kinetic energy of 1500 joule, an angular acceleration of 25radian/sec2 must be applied about the axis for duration of ___________
a) 4s
b) 2s
c) 8s
d) 10s

Explanation: Rotational kinetic energy = 1/2 Iω2
1500=1/2×1.2×ω2
t=(ω-ω0)/α=(50-0)/25
t=2sec.

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