CBSE Class 11 Physics Systems of Particles and Rotational Motion MCQs Set A

Question: The centre of mass of a body

a) lies always on the surface of the body

b) lies always inside the body

c) may lie within, outside on the surface of the body

d) lies always outside the body

Answer: lies always on the surface of the body

Question: The position of centre of mass of a system of particles does not depend upon the

a) relative distance between the particles

b) position of the particles

c) symmetry of the body

d) mass of particles

Answer: relative distance between the particles

Question: The motion of the centre of mass depends on

a) total external forces

b) total internal forces

c) None of these

d) Both

Answer: total external forces

Question: The centre of mass of two particles lies on the line

a) joining the particles

b) perpendicular to the line joining the particles

c) at any angle to this line

d) None of these

Answer: joining the particles

Question: In rotatory motion, linear velocities of all the particles of the body are

a) different

b) zero

c) cannot say

d) None of these

Answer: different

Question: If the resultant of all external forces is zero, then velocity of centre of mass will be

a) zero

b) neither (a) nor (b)

c) constant

d) either (a) or (b)

Answer: zero

Question: In rotation of a rigid body about a fixed axis, every ...A...of the body moves in a ...B..., which lies in a plane ...C... to the axis and has its centre on the axis. Here, A, B and C refer to

a) particle, circle and perpendicular

b) particle perpendicular and perpendicular

c) circle, particle and perpendicular

d) particle, perpendicular and circle

Answer: particle, circle and perpendicular

Question: Centre of mass of the earth and the moon system lies

a) closer to the earth

b) closer to the moon

c) at the mid-point of line joining the earth and the moon

d) cannot be predicted

Answer: closer to the earth

Question: A stick is thrown in the air and lands on the ground at some distance from the thrower. The centre of mass of the stick will move along a parabolic path

a) in all cases

b) only if the stick is uniform

c) only if the stick has linear motion but no rotational motion

d) only if the stick has a shape such that its centre of mass is located at some point on it and not outside it

Answer: in all cases

Question:  Assertion : The centre of mass of an isolated system has a constant velocity.

Reason : If centre of mass of an isolated system is already at rest, it remains at rest.

a) Assertion is correct, reason is correct; reason is not a correct explanation for assertion

b) Assertion is correct, reason is correct; reason is a correct explanation for assertion.

c) Assertion is incorrect, reason is correct

d) Assertion is correct, reason is incorrect

Answer: Assertion is correct, reason is correct; reason is not a correct explanation for assertion

Question: Concrete mixture is made by mixing cement, stone and sand in a rotating cylindrical drum. If the drum rotates too fast, the ingredients remain stuck to the wall of the drum and proper mixing of ingredients does not take place. The maximum rotational speed of the drum in revolutions per minute (rpm) to ensure proper mixing is close to : (Take the radius of the drum to be 1.25 m and its axle to be horizontal):

a) 27.0

b) 0.4

c) 1.3

d) 8.0

Answer: 27.0

Question: Angular velocity of each particle of a rotating rigid body about axis of rotation is

a) same

b) different

c) depends on relative position

d) None of these

Answer: same

Question: A wheel rotates with a constant acceleration of 2.0 radian/sec². If the wheel starts from rest, the number of revolutions it makes in the first ten seconds will be approximately

a) 16

b) 24

c) 32

d) 8

Answer: 16

Question: A wheel has a speed of 1200 revolutions per minute and is made to slow down at a rate of 4 radians/s². The number of revolutions it makes before coming to rest is

a) 314

b) 722

c) 272

d) 143

Answer: 314

Question: When a ceiling fan is switched off, its angular velocity falls to half while it makes 36 rotations. How many more rotations will it make before coming to rest?

a) 12

b) 18

c) 36

d) 24

Answer: 12

Question: A mass is revolving in a circle which is in the plane of paper. The direction of angular acceleration is

a) upward the radius

b) towards the radius

c) tangential

d) at right angle to angular velocity

Answer: upward the radius

Question: Which component of force contributes to the torque?

a) Transverse component

b) Either radial or transverse

c) Radial component

d) None of these

Answer: Transverse component

Question: A couple produces

a) rotational motion

b) linear motion

c) both

d) None of these

Answer: rotational motion

Question: Which of the following statements about angular momentum is correct?

a) It is directly proportional to moment of inertia

b) It is a scalar quantity

c) both

d) None of these

Answer: It is directly proportional to moment of inertia

Question: The wide handle of screw is based upon

a) turning moment of force

b) None of these

c) law of conservation of linear momentum

d) upon

Answer: turning moment of force

Question. A uniform circular disc of radius 50 cm at rest is free to turn about an axis which is perpendicular to its plane and passes through its centre. It is subjected to a torque which produces a constant angular acceleration of 2.0 rad s^–2. Its net acceleration in ms^–2 at the end of 2.0s is approximately :
(a) 8.0
(b) 7.0
(c) 6.0
(d) 3.0
Answer: a

Question. Two fly wheels A and B are mounted side by side with frictionless bearings on a common shaft. Their moments of inertia about the shaft are 5.0 kg m2 and 20.0 kg m² respectively. Wheel A is made to rotate at 10 revolution per second. Wheel B, initially stationary, is now coupled to A with the help of a clutch. The rotation speed of the wheels will become
(a) 2√5 rps
(b) 0.5 rps
(c) 2 rps
(d) None of these
Answer: c

Question. A tube one metre long is filled with liquid of mass 1 kg. The tube is closed at both the ends and is revolved about one end in a horizontal plane at 2 rev/s. The force experienced by the lid at the other end is
(a) 4π²N
(b) 8π²N
(c) 16π²N
(d) 9.8 N
Answer: b

Question. A weightless ladder 20 ft long rests against a frictionless wall at an angle of 60º from the horizontal. A 150 pound man is 4 ft from the top of the ladder. A horizontal force is needed to keep it from slipping. Choose the correct magnitude from the following.
(a) 175 lb
(b) 100 lb
(c) 120 lb
(d) 17.3 lb
Answer: d

Question. A coin placed on a gramophone record rotating at 33 rpm flies off the record, if it is placed at a distance of more than 16 cm from the axis of rotation. If the record is revolving at 66 rpm, the coin will fly off if it is placed at a distance not less than
(a) 1 cm
(b) 2 cm
(c) 3 cm
(d) 4 cm
Answer: d

Question. The moment of inertia of a body about a given axis is 1.2 kg m². Initially, the body is at rest . In order to produce a rotational kinetic energy of 1500 J, an angular acceleration of 25 rad s^–2 must be applied about that axis for a duration of
(a) 4 s
(b) 2 s
(c) 8 s
(d) 10 s
Answer: b

Question. If the linear density (mass per unit length) of a rod of length 3m is proportional to x, where x is the distance from one end of the rod, the distance of the centre of gravity of the rod from this end is
(a) 2.5 m
(b) 1 m
(c) 1.5 m
(d) 2 m
Answer: d

Question. A composite disc is to be made using equal masses of aluminium and iron so that it has as high a moment of inertia as possible. This is possible when
(a) the surfaces of the disc are made of iron with aluminium inside
(b) the whole of aluminium is kept in the core and the iron at the outer rim of the disc
(c) the whole of the iron is kept in the core and the aluminium at the outer rim of the disc
(d) the whole disc is made with thin alternate sheets of iron and aluminium
Answer: b

Question. Three identical particles each of mass 1 kg are placed touching one another with their centres on a straight line. Their centres are marked A, B and C respectively. The distance of centre of mass of the system from A is
(a) AB + AC + BC / 3
(b) AB + AC / 3
(c) AB + BC / 3
(d) AC + BC / 3
Answer: b

Question. In carbon monoxide molecule, the carbon and the oxygen atoms are separated by a distance 1.12 × 10^–10 m. The distance of the centre of mass, from the carbon atom is
(a) 0.64 × 10^–10 m
(b) 0.56 × 10^–10 m
(c) 0.51 × 10^–10 m
(d) 0.48 × 10^–10 m
Answer: a

Question. The moment of inertia of a disc of mass M and radius R about an axis, which is tangential to the circumference of the disc and parallel to its diameter, is
(a) 3/2 MR²
(b) 2/3 MR²
(c) 5/4 MR²
(d) 4/5 MR²
Answer: c