NCERT Solutions for Class 11 Physics Chapter 6 System Of Particles And Rotational Motion

NCERT Solutions for Class 11 Physics Chapter 6 System Of Particles And Rotational Motion

Vishal kumarUpdated on 18 Sep 2025, 12:50 AM IST

When a rubber ball is compressed, it alters its shape; when a metal rod is bent under pressure, it becomes deformed. In Physics, however, to make things simpler, we frequently consider such objects to be rigid bodies that neither change shape in response to forces exerted on them. With this assumption, we could simplify the study of their motion. Chapter 6, Class 11 Physics System of Particles and Rotational Motion presents students with some of the most significant concepts of mechanics, such as: torque, angular momentum, moment of inertia, and rotational dynamics. These principles are the foundation of the knowledge of motion and rotation of bodies in the real world.

This Story also Contains

  1. System of Particles and Rotational Motion Class 11 Question Answers: Download PDF
  2. System of Particles and Rotational Motion NCERT Solutions: Exercise Questions
  3. System of Particles and Rotational Motion NCERT Solutions: Higher Order Thinking Skills (HOTS) Questions
  4. Class 11 Physics Chapter 6 - System of Particles and Rotational Motion: Topics
  5. System of Particles and Rotational Motion NCERT Solutions: Key Formulas
  6. Approach to Solve Questions of Class 11 Physics Chapter 6 - System of Particles and Rotational Motion
  7. What Extra Should Students Study Beyond NCERT for JEE/NEET?
  8. Importance of NCERT Solutions for Class 11 Physics Chapter 6 - System of Particles and Rotational Motion
  9. NCERT Solutions for Class 11 Physics Chapter-Wise

The NCERT Solutions for Class 11 Physics Chapter 6 - System of Particles and Rotational Motion are well organised and prepared by subject experts so that it can give a step-by-step explanation of all concepts, derivations, formulas and numerical problems. These resources, which contain NCERT solutions to in-text and exercise questions in detail, simplify even complex concepts further and allow understanding and application. The chapter is very important not only to CBSE board exams but also to competitive exams such as JEE and NEET, where questions on rotational motion are often asked. Regular practice with NCERT Solutions for Class 11 Physics Chapter 6 - System of Particles and Rotational Motion assists the students to improve upon their basics, perform well in school examinations, and gain a strong foundation to continue their education in physics.

System of Particles and Rotational Motion Class 11 Question Answers: Download PDF

The Class 11 Physics Chapter 6 - System of Particles and Rotational Motion question answers provide solutions to all the questions in the book, and teach students what the centre of mass is, how torque and angular momentum work, and what determines the rotational dynamics. The PDF version can be downloaded and works well in helping to revise as well as prepare for both board exams and competitive exams like JEE and NEET.

Download Solution PDF

System of Particles and Rotational Motion NCERT Solutions: Exercise Questions

System of Particles and Rotational Motion class 11 question answers enable the students to understand the important concepts such as centre of mass, torque, angular momentum and rotational dynamics. These are step-wise solutions that will help understand concepts, sharpen problem-solving and also equip students for school tests and competitive exams such as JEE and NEET.

Q6.1 Give the location of the centre of mass of a (i) sphere, (ii) cylinder, (iii) ring, and (iv) cube, each of uniform mass density. Does the centre of mass of a body necessarily lie inside the body?

Answer:

For uniform mass density, the location of the centre of mass is the same as that of the geometric centre.

No, it is not necessary that the centre of mass lies inside the body. For example, in the case of a ring, the centre of mass is outside the body.

Q6.2 In the HCl molecule, the separation between the nuclei of the two atoms is about 1.27Å(1Å=1010m) . Find the approximate location of the CM of the molecule, given that a chlorine atom is about 35.5 times as massive as a hydrogen atom and nearly all the mass of an atom is concentrated in its nucleus.

Answer:

Let us assume that the centre of mass of the molecule is x cm away from the chlorine atom.

So, we can write

m(1.27 x) + 35.5mxm + 35.5 = 0

or x = 1.27(35.51) = 0.037Å

Here negative sign indicates that the centre of mass lies leftward of the chlorine atom.

Q6.3 A child sits stationary at one end of a long trolley moving uniformly with a speed v on a smooth horizontal floor. If the child gets up and runs about on the trolley in any manner, what is the speed of the CM of the (trolley + child) system?

Answer:

Since the child and trolley are considered in a system, the speed of the centre of mass changes only when an external force acts on the system. When the child starts to move on the trolley, the force produced is the internal force of the system, so this will produce no change in the velocity of the CM.

Q6.4 Show that the area of the triangle contained between the vectors aandb is one-half of the magnitude of a×b .

Answer:

Let a and b be two vectors having an angle θ between them.

Consider Δ MON,

sinθ = MNMO

or sinθ = MN|b|

or MN = bsinθ

|a×b| = |a||b|sinθ= 2 (Area of Δ MOK)

Therefore, the area of Δ MOK = one-half of |a×b|.

Q6.5 Show that a.(b×c) is equal in magnitude to the volume of the parallelepiped formed on the three vectors , a , b and c.

Answer:

A parallelepiped is shown in the figure given below:-

Volume is given by: abc

We can write :

|b×c| = |b||c|sinθ n^ (The direction of n^ is in the direction of vector a)

|b×c|= |b||c|sin90 n^

|b×c|= |b||c| n^

Now,

a.(b×c) = a.(bc) n^

= abccosθ

= abccos0

= abc

This is equal to the volume of a parallelepiped.

Q6.6 Find the components along the x,y,z axes of the angular momentum 1 of a particle, whose position vector is r with components x,y,z and momentum is p with components px , py and pz . Show that if the particle moves only in the xy plane the angular momentum has only a z-component.

Answer:

Linear momentum of a particle is given by :

p = pxi^ + pyj^ + pzk^

And the angular momentum is :

l =r×p

l =(xi^ + yj^ + zk^)×(pxi^ + pyj^ + pzk^)

l =|ijkxyzPxPyPz|

l = i^(ypzzpy) j^(xpzzpx) + k^(xpyypx)

When a particle is confined to the x-y plane, then z = 0 and pz = 0.

When we put the value of z and p z in the equation of linear momentum, then we observe that only the z component is non-zero.

Q6.7 Two particles, each of mass m and speed v , travel in opposite directions along parallel lines separated by a distance d . Show that the angular momentum vector of the two particle system is the same whatever be the point about which the angular momentum is taken.

Answer:

Assume two points (say A, B) separated by distance d.

So the angular momentum of a point about point A is given by: = mv×d = mvd

And about point B : = mv×d = mvd

Now, assume a point between A and B as C, which is at a distance y from point B.

Now the angular momentum becomes : = mv×(dy) + mv×y= mvd

Thus, it can be seen that angular momentum is independent of the point about which it is measured.

Q6.8 A non-uniform bar of weight W is suspended at rest by two strings of negligible weight as shown in Fig. The angles made by the strings with the vertical are 36.9o and 53.1o respectively. The bar is 2m long. Calculate the distance d of the centre of gravity of the bar from its left end.

Answer:

The FBD of the given bar is shown below :

Since the bar is in equilibrium, we can write :

T1sin36.9 = T2sin53.1

or T1T2 = 0.8000.600 = 43

or T1 = 43T2 .....................................................(i)

For the rotational equilibrium :

T1cos36.9×d = T2cos53.1×(2d) (Use equation (i) to solve this equation)

or d = 1.21.67 = 0.72 m

Thus, the centre of gravity is at 0.72 m from the left.

Q6.9 A car weighs 1800kg . The distance between its front and back axles is 1.8m . Its centre of gravity is 1.05m behind the front axle. Determine the force exerted by the level ground on each front wheel and each back wheel.

Answer:

The FBD of the car is shown below :

We will use the conditions of equilibrium here :

Rf + Rb = mg

1800×9.8 = 17640 N ....................................(i)

For rotational equilibrium :

Rf(1.05) = Rb(1.81.05)

or 1.05Rf = 0.75Rb

Rb = 1.4Rf ..............................(ii)

From (i) and (ii), we get :

Rf = 176402.4 = 7350 N

and Rb = 176407350 = 10290 N

Thus force exerted by the front wheel is = 3675 N

and force exerted bythe back wheel = 5145 N.

Answer:

The moment of inertia of a hollow cylinder is given by = mr2

And the moment of inertia of a solid cylinder is given by: = 25mr2

We know that: τ = Iα

Let the torque for the hollow cylinder be τ1, and for the solid cylinder let it be τ2.

According to the question: τ1 = τ2

So we can write the ratio of the angular acceleration of both objects.

α2α1 = I1I2 = mr225mr2 = 25

Now for angular velocity,

ω = ωo + αt

Clearly, the angular velocity of the solid sphere is more than the angular velocity of the hollow sphere. (As the angular acceleration of a solid sphere is greater).

Q6.11 A solid cylinder of mass 20kg rotates about its axis with angular speed 100rads1. The radius of the cylinder is 0.25m . What is the kinetic energy associated with the rotation of the cylinder? What is the magnitude of the angular momentum of the cylinder about its axis?

Answer:

Firstly, we will calculate the moment of inertia of the solid cylinder:

Ic= 12mr2

Ic= 12(20)(0.25)2 = 0.625 Kg m2

So the kinetic energy is given by :

Ek=12Iω2=12×(6.25)×(100)2=3125J

And the angular momentum is given by: =Iω=0.625×100=62.5Js

Q6.12 (a) A child stands at the centre of a turntable with his two arms outstretched. The turntable is set rotating with an angular speed of 40rev/min . How much is the angular speed of the child if he folds his hands back and thereby reduces his moment of inertia to 2/5times the initial value? Assume that the turntable rotates without friction.

Answer:

We are given the initial angular speed and the relation between the moment of inertia of both cases.

Here we can use conservation of angular momentum as no external force is acting on the system.

So we can write :

I1w1 = I2w2

w2=I1w1I2=I(40)25I

w2= 100 rev/min

Q6.12(b) Show that the child’s new kinetic energy of rotation is more than the initial kinetic energy of rotation. How do you account for this increase in kinetic energy?

Answer:

The final and initial velocities are given below :

Ef = 12I2w22 and Ei = 12I1w12

Taking the ratio of both, we get,

EfEi = 12I2w2212I1w12

or EfEi= 25×100×10040×40

or EfEi= 2.5

Thus, the final energy is 2.5 times the initial energy.

The increase in energy is due to the internal energy of the boy.

Q6.13 A rope of negligible mass is wound round a hollow cylinder of mass 3kg and radius 40cm . What is the angular acceleration of the cylinder if the rope is pulled with a force of 30N ? What is the linear acceleration of the rope? Assume that there is no slipping.

Answer:

The moment of inertia is given by :

I = mr2

or I= 3×(0.4)2 = 0.48 Kg m2

And the torque is given by :

τ = r×F= 0.4×30 = 12 Nm

Also, τ = Iα

So α = τI = 120.48 = 25 rad/s2

And the linear acceleration is a = αr = 0.4×25 = 10 m/s2

Q6.15 From a uniform disk of radius R , a circular hole of radius R/2 is cut out. The centre of the hole is at R/2 from the centre of the original disc. Locate the centre of gravity of the resulting flat body.

Answer:

Let the mass per unit area of the disc be σ.

So total mass = πr2σ = m

Mass of the smaller disc = π(r2)2σ = m4

Now, since the disc is removed, we can assume that part to have negative mass with respect to the initial condition.

So, the centre of mass of the disc is given by the formula :

x =m1r1 + m2r2m1 + m2

Or x= m×0 m4×r2m m4

Or, x= r6

Hence, the centre of mass is shifted r6 leftward from point O.

Q6.16 A metre stick is balanced on a knife-edge at its centre. When two coins, each of mass 5g are put one on top of the other at the 12.0cm mark, the stick is found to be balanced at 45.0cm . What is the mass of the metre stick?

Answer:

The centre of mass of the meter stick is at 50 cm.

Let the mass of the meter stick be m.

Now, according to the situation given in the question, we will use the rotational equilibrium condition at the centre point of the meter stick.

10g(4512) mg(5045) = 0

or m = 10×335 = 66 g

Thus, the mass of the meter stick is 66 g.

Q6.17 The oxygen molecule has a mass of 5.30×1026kg and a moment of inertia of 1.94×1046kgm2 about an axis through its centre perpendicular to the lines joining the two atoms. Suppose the mean speed of such a molecule in a gas is 500m/s and that its kinetic energy of rotation is two-thirds of its kinetic energy of translation. Find the average angular velocity of the molecule.

Answer:

We are given the moment of inertia and the velocity of the molecule.

Let the mass of an oxygen molecule be m.

So the mass of each oxygen atom is given by: m2

Moment of inertia is :

I = m2r2 + m2r2 = mr2

or r = Im

or r = 1.94×10465.36×1026 = 0.60×1010 m

We are given that :

Erot = 23Etra

or 12Iω2 = 23×12mv2

or ω = 23×vr

or ω= 6.80×1012 rad/s

System of Particles and Rotational Motion NCERT Solutions: Higher Order Thinking Skills (HOTS) Questions

The Higher Order Thinking Skills (HOTS) questions of Class XII Physics Chapter 6: System of Particles and Rotational Motion are developed in such a way that they enhance theoretical knowledge, as well as assess the application of learning. These questions can be much more than simple problem-solving ones, and invite the students to analyse, evaluate and apply concepts of rotational motion and centre of mass to new and complicated scenarios, which are perfect to practice a competitive exam such as JEE.

Question 1) A solid sphere of mass M and radius R is rolling without slipping with a velocity v0 on a horizontal surface. It encounters an inclined plane making an angle θ with the horizontal. Assuming no energy loss due to friction, find the maximum height h the sphere will reach before coming to rest.
1) 5v026q2

2) 3v024g

3) 7v0210g

4) v022g

Answer:

For a rolling solid sphere, the total kinetic energy is the sum of translational and rotational kinetic energy:

Etotal =KEtranslational +KErotational


KEtotal =12Mv02+12Iω2


For a solid sphere, the moment of inertia about its centre is:

I=25MR2


Using the rolling without slipping condition, v0=Rω, so:

ω=v0R


Substituting I and ω :

KErotational =12×25MR2×(v0R)2=15Mv02


So, the total kinetic energy:

KEtotal =12Mv02+15Mv02


KEtotal =510Mv02+210Mv02=710Mv02


At maximum height, all kinetic energy converts into gravitational potential energy:

PE=Mgh

Equating energy:

710Mv02=Mghh=7v0210g

Hence, the answer is the option (3).

Question 2) A rod starts rotating with angular velocity ωo but due to viscous force, its angular speed starts decreasing with variable retardation: α=cω Find out the angle traversed by the rod as a function of time.
1) θ=2ω0c[1ect]

2) θ=ω0c[1ect]

3) θ=ω02c[1ect]

4) θ=ω03c[1ect]

Answer:

We know

α=cωdωdt=cωa0ωdωω=0tcdtlnωω0=ctω=ω0ectdθdt=ω0ect0θdθ=ω00tectdtθ=ω0c[1ect]
Hence, the answer is the option (2).

Question 3) A wedge of mass M=2kg carries two blocks A and B, as shown in the figure. Block A lies on a smooth-incline surface of the wedge inclined at an angle θ to the horizontal. B lies inside a horizontal groove made in the wedge. The two blocks have m=1kg each and are connected by a light string. Length of the groove is l=2m as shown. The entire system is released from rest. What is the distance travelled by the wedge (in m) on the smooth horizontal surface by the time block B comes out of the groove?

Answer:

Let the displacement of the wedge be x towards the right. Displacement of B=(x1) towards the right.
Displacement of A w.r.t wedge (on the incline) is l.
Displacement of A in horizontal direction =lcosθ+x
The COM of the entire system suffers no displacement in the horizontal direction.

ΔxCOM=0m1Δx1+m2Δx2+m3Δx3=0Mx+m(xl)+m(lcosθ+x)=0x=ml(1cosθ)M+2m
Now, putting m=1 kg,M=2kgI=2 m and θ=60 in (i), we get

x=1×2(1cos60)2+2×1x=0.25 m
Hence, the answer is 0.25.

Question 4)

A sphere is released on a smooth inclined plane from the top. When it moves down, its angular momentum is:
1) Conserved about every point
2) Conserved about the point of contact only
3) Conserved about the centre of the sphere only
4) Conserved about any point on a line parallel to the inclined plane and passing through the centre of the ball.

Answer:

As the inclined plane is smooth, the sphere can never roll; rather, it will just slip down. Hence, the angular momentum remains conserved at any point on a line parallel to the inclined plane and passing through the centre of the ball.

Hence, the answer is the option (4).

Question 5) A square plate of mass m and length l is rotated with angular velocity ' w ' with a vertical axis passing through its centre, and it is kept on a rough surface. It takes ' t0 ' time to stop because of uniform friction. Now, if we hinge it at the corner & give the same angular velocity, In kt0 time it will take to stop, Find k.

Answer:

The torque by friction will be τ = Cμmgl, and the moment of inertia will be,I=kml2

τ= Iα

Hence, angular acceleration will be

α=Cμgk(1l)

α is inversely proportional to l

Now, as it is hinged at one corner, we can assume a plate of double-sided length

Now this is again the central axis, the value of angular acceleration will be half.

Hence, it will take 2t0 hours to stop.

Hence, the answer is 2.

Class 11 Physics Chapter 6 - System of Particles and Rotational Motion: Topics

In Class 11 Physics Chapter 6 - System of Particles and Rotational Motion, the topics that are covered deal with centre of mass, motion of system of particles, torque, angular velocity, angular momentum and rotational dynamics. These topics constitute the basis of learning complex physical systems and are of great value in board exams and competitive exams such as JEE and NEET.

6.1 Introduction
6.1.1 What Kind Of Motion Can A Rigid Body Have?
6.2 Centre Of Mass
6.3 Motion Of the Centre Of Mass
6.4 Linear Momentum Of A System Of Particles
6.5 Vector Product Of Two Vectors
6.6 Angular Velocity And Its Relation With Linear Velocity
6.6.1 Angular Acceleration
6.7 Torque And Angular Momentum
6.7.1 Moment Of Force (Torque)
6.7.2 Angular Momentum Of A Particle
6.8 Equilibrium Of A Rigid Body
6.8.2 Centre Of Gravity
6.9 Moment Of Inertia
6.10 Kinematics Of Rotational Motion About A Fixed Axis
6.11 Dynamics Of Rotational Motion About A Fixed Axis
6.12 Angular Momentum In Case Of Rotation About A Fixed Axis
6.12.1 Conservation Of Angular Momentum

System of Particles and Rotational Motion NCERT Solutions: Key Formulas

System of Particles and Rotational Motion class 11 question answers: Key Formulas include important equations related to centre of mass, torque, moment of inertia, angular momentum, and rotational kinetic energy. These formulas act as quick revision tools and help students solve numerical problems efficiently in exams like CBSE, JEE, and NEET.

1. Centre of mass

For a system of particles: RCOM=mirimi

2. Motion of COM

M=mi
vcm=miviM

pcm=Mvcm

acm=FextM

3. Angular Velocity & Linear Velocity

Average angular velocity =∆θ/∆t

Instantaneous angular velocity, ω=dθ/dt

4. Moment of Inertia (MI)

I=miri2 (Discrete system) I=r2dm (Continuous body)

5. Moment of Inertia of a few useful configurations

6. Torque (Moment of Force)

τ=r×F=rFsinθ

7. Angular Momentum

L=r×p=mr×v
For a rotating body:

L=Iω

8. Torque-Angular Acceleration Relation

τ=Iα

9. Rotational Kinetic Energy

KErot=12Iω2

10. Angular Equations of Motion (Uniform Angular Acceleration)

ω=ω0+αtθ=ω0t+12αt2ω2=ω02+2αθ

Approach to Solve Questions of Class 11 Physics Chapter 6 - System of Particles and Rotational Motion

In Physics, Chapter 11, System of Particles and Rotational Motion, Chapter 6 is concerned with the motion of rigid bodies, torque, angular momentum and moment of inertia. This chapter requires conceptual clarity and practice in problem-solving methods to address the questions effectively in this chapter. The systematic approach can be used to make sure that there are no inaccuracies in derivations, numericals, and applications.

  • Knowledge of Rigid Body Assumption: It is always good to start by noting that the chapter assumes that the bodies are rigid (no deformation). This makes the problems of torque and rotational motion easier.
  • Divide Problems into Translational and Rotational: When solving, separate linear motion (using Newton's laws) and rotational motion (using torque = Iα). They should only be combined when necessary.
  • Use Vector Representation: Appropriate rules of vectors should be applied to answer questions on torque, angular momentum and cross products. Practice right-hand rule applications for direction.
  • Learn and Use Theorems of Inertia: The theorems which are commonly applied in numerical questions are the parallel axis and the perpendicular axis theorems. Use them directly in practice in order to save time during exams.
  • Study Motion About the Center of Mass (COM): In problem of particle systems find the COM first. Simplification can then be made on motion of the entire system by taking translation + rotation of a system about the COM.
  • Focus on Derivations and Proofs: Derivations such as the correlation between torque and angular momentum, kinetic energy in rotation and rolling motion are common exam problems to be prepared.
  • Connect with Real-Life Applications: Comparing rotational motion to real-life examples such as wheels, flywheels, ceiling fans or gyroscopes can be beneficial when it comes to writing a better response to an explanation.

What Extra Should Students Study Beyond NCERT for JEE/NEET?

To crack JEE, one must get out of NCERT and must practice higher level conceptual problems of torque, rotational motion of Chapter 6: System of Particles and Rotational Motion and rolling motion. Answering difficult questions and practising past year JEE questions will enable you to brush through the depth of knowledge needed to crack competitive exams.

Importance of NCERT Solutions for Class 11 Physics Chapter 6 - System of Particles and Rotational Motion

Rotational motion is a concept that students find difficult to understand because it entails the use of abstract concepts and mathematics. In an effort to eliminate these challenges, Class 11 Physics Chapter 6 - System of Particles and Rotational Motion question answers could be a good source of help that will provide a clear explanation and the correct answers. These solutions are useful not only in explaining how to solve the problems step by step but also in bringing out the key formulas and derivations that are very important in exams. They serve as the mediator between theory and practice making sure that students are both precise and quick in answering questions.

  • Conceptual Clarity: The solutions provide a clear and detailed explanation of important concepts like the centre of mass, moment of inertia, and rotational motion, helping students develop a strong conceptual foundation.
  • Step-by-Step Solutions: Each problem is solved step by step, making it easier for students to understand the methods and approaches used to tackle complex problems.
  • Boosts Problem-Solving Skills: By practising the problems from the solutions, students improve their analytical and problem-solving skills, which are essential for both board exams and competitive tests.
  • Reinforces Core Concepts: The solutions help reinforce key topics such as Newton's laws in rotational motion, torque, and angular momentum, which are essential for mastering the chapter.
  • Effective Exam Preparation: These solutions are designed to help students prepare efficiently for exams by covering all important topics and providing a strong grasp of the subject.
  • Quick Revision: The solutions are concise and well-organized, making them a great resource for quick revision before exams.

In summary, the NCERT solutions for Class 11 Physics help students effectively understand and solve problems in the chapter, strengthening their knowledge and enhancing their performance in exams.

Also, Check NCERT Books and NCERT Syllabus here

NCERT Solutions for Class 11 Physics Chapter-Wise

The NCERT Solutions to Class 11 Physics are available in chapter-wise format to have inexpensive, step-by-step solutions to each chapter. The links are useful in revising major concepts, doing numerical problems, and preparing a sound base on exams such as CBSE, JEE, and NEET. All solutions are designed by professionals according to the new syllabus.

NCERT solutions for class 11 Subject-wise

Frequently Asked Questions (FAQs)

Q: What is the primary topic in this chapter?
A:

It is about constraint on the motion of rigid bodies, rotation about a fixed axis and concepts such as torque, moment of inertia and angular momentum.

Q: Why do we assume rigid bodies in this chapter?
A:

By assuming rigidity, it eliminates complications of deformation hence making the analysis easy.

Q: What are the differences between rotational motion and linear motion?
A:

Linear motion is the movement in a straight direction and rotational is the turning around an axis.

Q: Is moment of inertia equal to mass in rotation?
A:

Yes it bears the same role in rotational motion as mass does in linear motion.

Q: What are some of the real world examples of rotational motion?
A:

The most common example includes rotating wheels, ceiling fans, spinning tops, and rotating planets.

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