When you apply some force on a rubber ball, it deforms and becomes distorted; a metal rod can become curved when subjected to sufficient pressure. However, in Physics, in order to simplify the solution of a particular problem, we tend to set a context where the objects behave like rigid bodies, i.e. they do not change their deformations under set forces, particularly when the deformations are negligible and do not have noticeable effects on the behaviour of the object.
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Chapter 6 of the Class 11 Physics, System of Particles and Rotational Motion, presents the students with a combination of complex and abstract concepts such as the torque, angular momentum, moment of inertia, and rotational dynamics. These ideas are fundamental to the motion and rotation of bodies in the real world. The NCERT Solutions are well prepared with the help of experienced teachers to help students learn this chapter. The NCERT solutions of Class 11 Physics provide each concept in steps so that one can easily understand the derivations, formulas and numerical applications. This chapter is very important and reading material, and attempts at repetitive questions are essential to get good marks in school examinations and in competitive tests such as JEE and NEET.
The Systems of Particles and Rotational Motion chapter provides solutions to all the questions in the book, and teaches students what the center 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.
NCERT Solutions Class 11 Physics Chapter 7: System of Particles and Rotational Motion enables 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.
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 outside the body.
Answer:
Let us assume that the centre of mass of the molecule is x cm away from the chlorine atom.
So, we can write
$\frac{m\left ( 1.27\ -\ x \right )\ +\ 35.5mx}{m\ +\ 35.5 }\ =\ 0$
or $x\ =\ \frac{-1.27}{\left ( 35.5-1 \right )}\ =\ -\ 0.037 Å$
Here negative sign indicates that the centre of mass lies leftward of the chlorine atom.
Answer:
Since the child and trolley are considered in a system, the speed of the centre of mass change only when an external force will act 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.
Answer:
Let a and b be two vectors having $\theta$ angle between them.
Consider $\Delta$ MON,
$\sin \theta \ =\ \frac{MN}{MO}$
or $\sin \theta \ =\ \frac{MN}{|\vec{b}|}$
or $MN\ =\ b\sin \theta$
$\left | \vec{a}\times \vec{b} \right |\ =\ \left | \vec{a} \right |\left | \vec{b} \right |\sin \theta$= 2 (Area of $\Delta$ MOK)
Therefore the area of $\Delta$ MOK = One half of $\left | \vec{a}\times \vec{b} \right |$ .
Answer:
A parallelepiped is shown in the figure given below:-
Volume is given by : abc
We can write :
$|\vec{b}\times \vec{c}|\ =\ |\vec{b}||\vec{c}|\sin \theta\ \widehat{n}$ (The direction of $\widehat{n}$ is in the direction of vector $\vec{a}$)
$\Rightarrow |\vec{b}\times \vec{c}|=\ |b||c|\sin 90^{\circ}\ \widehat{n}$
$\Rightarrow |\vec{b}\times \vec{c}|=\ |b||c|\ \widehat{n}$
Now,
$\vec{a}.(\vec{b}\times \vec{c})\ =\ \vec{a}.(bc)\ \widehat{n}$
$=\ abc \cos \theta$
$=\ abc \cos 0^{\circ}$
$=\ abc$
This is equal to volume of parallelepiped.
Q6.6 Find the components along the $x,y,z$ axes of the angular momentum $1$ of a particle, whose position vector is $\vec{r}$ with components $x,y,z$ and momentum is $\vec{p}$ with components $p_{x}$ , $p_{y}$ and $p_{z}$ . Show that if the particle moves only in the $x-y$ plane the angular momentum has only a z-component.
Answer:
Linear momentum of particle is given by :
$\overrightarrow{p}\ =\ p_x \widehat{i}\ +\ p_y \widehat{j}\ +\ p_z \widehat{k}$
And the angular momentum is :
$\overrightarrow{l}\ = \overrightarrow{r}\times \overrightarrow{p}$
$\Rightarrow \overrightarrow{l}\ = \left ( x \widehat{i}\ +\ y \widehat{j}\ +\ z \widehat{k} \right ) \times \left ( p_x \widehat{i}\ +\ p_y \widehat{j}\ +\ p_z \widehat{k} \right )$
$\Rightarrow \overrightarrow{l}\ =\begin{vmatrix} i &j &k \\ x &y &z \\ P_x &P_y &P_z \end{vmatrix}$
$\Rightarrow \overrightarrow{l}\ =\ \widehat{i}\left ( yp_z - zp_y \right )\ -\ \widehat{j}\left ( xp_z - zp_x \right )\ +\ \widehat{k}\left ( xp_y - yp_x \right )$
When particle is confined to 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.
Answer:
Assume two points (say A, B) separated by distance d.
So the angular momentum of a point about point A is given by : $=\ mv \times d\ =\ mvd$
And about point B : $=\ mv \times d\ =\ mvd$
Now assume a point between A and B as C which is at y distance from point B.
Now the angular momentum becomes : $=\ mv \times \left ( d-y \right )\ +\ mv \times y=\ mv d$
Thus it can be seen that angular momentum is independent of the point about which it is measured.
Answer:
The FBD of the given bar is shown below :
Since the bar is in equilibrium, we can write :
$T_1 \sin 36.9 ^{\circ}\ =\ T_2 \sin 53.1 ^{\circ}$
or $\frac{T_1}{T_2}\ =\ \frac{0.800}{0.600}\ =\ \frac{4}{3}$
or $T_1\ =\ \frac{4}{3}T_2$ .....................................................(i)
For the rotational equilibrium :
$T_1 \cos 36.9 ^{\circ}\times d\ =\ T_2 \cos 53.1 ^{\circ}\times \left ( 2-d \right )$ (Use equation (i) to solve this equation)
or $d\ =\ \frac{1.2}{1.67}\ =\ 0.72\ m$
Thus the center of gravity is at 0.72 m from the left.
Answer:
The FBD of the car is shown below :
We will use conditions of equilibrium here :
$R_f\ +\ R_b\ =\ mg$
$\Rightarrow \ 1800\times 9.8\ =\ 17640\ N$ ....................................(i)
For rotational equilibrium :
$R_f\left ( 1.05 \right )\ =\ R_b(1.8-1.05)$
or $1.05R_f\ =\ 0.75R_b$
$R_b\ =\ 1.4R_f$ ..............................(ii)
From (i) and (ii) we get :
$R_f\ =\ \frac{17640}{2.4}\ =\ 7350\ N$
and $R_b\ =\ 17640-7350\ =\ 10290\ N$
Thus force exerted by the front wheel is = 3675 N
and force exerted by back wheel = 5145 N.
Answer:
The moment of inertia of hollow cylinder is given by $=\ mr^2$
And the moment of inertia of solid cylinder is given by : $=\ \frac{2}{5}mr^2$
We know that : $\tau \ =\ I\alpha$
Let the torque for hollow cylinder be $\tau_1$ and for solid cylinder let it be $\tau_2$ .
According to the question : $\tau_1\ =\ \tau_2$
So we can write the ratio of the angular acceleration of both the objects.
$\frac{\alpha _2}{\alpha _1}\ =\ \frac{I_1}{I_2}\ =\ \frac{mr^2}{\frac{2}{5}mr^2}\ =\ \frac{2}{5}$
Now for angular velocity,
$\omega \ =\ \omega _o\ +\ \alpha t$
Clearly, the angular velocity of the solid sphere is more than the angular velocity of the hollow sphere. (As the angular acceleration of solid sphere is greater).
Answer:
Firstly we will calculate moment of inertia of the solid cyliner :
$I_c =\ \frac{1}{2}mr^2$
$\Rightarrow I_c=\ \frac{1}{2}(20)(0.25)^2\ =\ 0.625\ Kg\ m^2$
So the kinetic energy is given by :
$E_k = \frac{1}{2}I\omega ^2= \frac{1}{2}\times (6.25)\times (100) ^2=3125J$
And the angular momentum is given by : $= I\omega= 0.625\times 100= 62.5 Js$
Answer:
We are given with the initial angular speed and the relation between the moment of inertia of both the cases.
Here we can use conservation of angular momentum as no external force is acting the system.
So we can write :
$I_1w_1\ =\ I_2w_2$
$w_2 = \frac{I_1w_1}{I_2}= \frac{I(40)}{\frac{2}{5}I}$
$w_2 =\ 100\ rev/min$
Answer:
The final and initial velocities are given below :
$E_f\ =\ \frac{1}{2}I_2w_2^2$ and $E_i\ =\ \frac{1}{2}I_1w_1^2$
Taking the ratio of both we get,
$\frac{E_f}{E_i}\ =\ \frac{\frac{1}{2}I_2w_2^2}{\frac{1}{2}I_1w_1^2}$
or $\frac{E_f}{E_i}=\ \frac{2}{5}\times \frac{100\times 100}{40\times40}$
or $\frac{E_f}{E_i}=\ 2.5$
Thus the final energy is 2.5 times the initial energy.
The increase in energy is due to the internal energy of the boy.
Answer:
The moment of inertia is given by :
$I\ =\ mr^2$
or $I=\ 3\times (0.4)^2\ =\ 0.48\ Kg\ m^2$
And the torque is given by :
$\tau \ =\ r\times F=\ 0.4\times 30\ =\ 12\ Nm$
Also, $\tau\ =\ I\alpha$
So $\alpha \ =\ \frac{\tau }{I}\ =\ \frac{12}{0.48}\ =\ 25\ rad/s^{-2}$
And the linear acceleration is $a\ =\ \alpha r\ =\ 0.4\times 25\ =\ 10\ m/s^{-2}$
Answer:
The relation between power and torque is given by :
$P\ =\ \tau \omega$
or $P=\ 180\times 200\ =\ 36000\ W$
Hence the required power is 36 KW.
Answer:
Let the mass per unit area of the disc be $\sigma$ .
So total mass $=\ \pi r^2\sigma\ =\ m$
Mass of the smaller disc $=\ \pi \left ( \frac{r}{2} \right )^2\sigma\ =\ \frac{m}{4}$
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 :
$x\ = \frac{m_1r_1\ +\ m_2r_2}{m_1\ +\ m_2}$
Or $x=\ \frac{m\times 0\ -\ \frac{m}{4}\times \frac{r}{2}}{m\ -\ \frac{m}{4}}$
Or, $x=\ \frac{-r}{6}$
Hence the centre of mass is shifted $\frac{r}{6}$ leftward from point O.
Answer:
The centre of mass of meter stick is at 50 cm.
Let the mass of 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.
$10g(45 - 12)\ -\ mg(50 - 45)\ =\ 0$
or $m\ =\ \frac{10\times 33}{5}\ =\ 66\ g$
Thus the mass of the meter stick is 66 g.
Answer:
We are given the moment of inertia and the velocity of the molecule.
Let the mass of oxygen molecule be m.
So the mass of each oxygen atom is given by : $\frac{m}{2}$
Moment of inertia is :
$I\ =\ \frac{m}{2}r^2\ +\ \frac{m}{2}r^2\ =\ mr^2$
or $r\ =\ \sqrt{\frac{I}{m}}$
or $r\ =\ \sqrt{\frac{1.94\times 10^{-46}}{5.36\times 10^{-26}}}\ =\ 0.60\times 10^{-10}\ m$
We are given that :
$E_{rot}\ =\ \frac{2}{3}E_{tra}$
or $\frac{1}{2}I\omega ^2\ =\ \frac{2}{3}\times \frac{1}{2}mv^2$
or $\omega \ =\ \sqrt{\frac{2}{3}}\times \frac{v}{r}$
or $\omega=\ 6.80\times 10^{12}\ rad/s$
The Higher Order Thinking Skills (HOTS) questions of Class XII Physics Chapter 6: System of Particles and Rotational Motion is developed in such a way that it enhances theoretical knowledge, as well as assesses the application learning. These questions can be much more than simple problem solving ones, and invite the students to analyze, evaluate and apply concepts of rotational motion and center 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 $v_0$ on a horizontal surface. It encounters an inclined plane making an angle $\theta$ with the horizontal. Assuming no energy loss due to friction, find the maximum height $h$ the sphere will reach before coming to rest.
1) $
\frac{5 v_0^2}{6 q_2}
$
2) $
\frac{3 v_0^2}{4 g}
$
3) $
\frac{7 v_0^2}{10 g}
$
4) $
\frac{v_0^2}{2 g}
$
Answer:
For a rolling solid sphere, the total kinetic energy is the sum of translational and rotational kinetic energy:
$
E_{\text {total }}=K E_{\text {translational }}+K E_{\text {rotational }} \\$
$K E_{\text {total }}=\frac{1}{2} M v_0^2+\frac{1}{2} I \omega^2
$
For a solid sphere, the moment of inertia about its center is:
$
I=\frac{2}{5} M R^2
$
Using the rolling without slipping condition, $v_0=R \omega$, so:
$
\omega=\frac{v_0}{R}
$
Substituting $I$ and $\omega$ :
$
\begin{aligned}
K E_{\text {rotational }} & =\frac{1}{2} \times \frac{2}{5} M R^2 \times\left(\frac{v_0}{R}\right)^2 \\
& =\frac{1}{5} M v_0^2
\end{aligned}
$
So, the total kinetic energy:
$
K E_{\text {total }}=\frac{1}{2} M v_0^2+\frac{1}{5} M v_0^2 \\$
$K E_{\text {total }}=\frac{5}{10} M v_0^2+\frac{2}{10} M v_0^2=\frac{7}{10} M v_0^2
$
At maximum height, all kinetic energy converts into gravitational potential energy:
$
P E=M g h
$
Equating energy:
$
\begin{gathered}
\frac{7}{10} M v_0^2=M g h \\
h=\frac{7 v_0^2}{10 g}
\end{gathered}
$
Hence, the answer is the option (3).
Question 2) A rod starts rotating with angular velocity $\omega_o$ but due to viscous force, its angular speed starts decreasing with variable retardation: $\alpha=-c \omega$ Find out the angle traversed by the rod as a function of time.
1) $
\theta=2 \frac{\omega_0}{c}\left[1-e^{-c t}\right]
$
2) $
\theta=\frac{\omega_0}{c}\left[1-e^{-c t}\right]
$
3) $
\theta=\frac{\omega_0}{2 c}\left[1-e^{-c t}\right]
$
4) $
\theta=\frac{\omega_0}{3 c}\left[1-e^{-c t}\right]
$
Answer:
We know
$
\begin{aligned}
& \alpha=-c \omega \\
& \frac{d \omega}{d t}=-c \omega \\
& \Rightarrow \int_{a_0}^\omega \frac{d \omega}{\omega}=-\int_0^t c d t \\
& \ln \frac{\omega}{\omega_0}=-c t \\
& \Rightarrow \omega=\omega_0 e^{-c t} \\
& \Rightarrow \quad \frac{d \theta}{d t}=\omega_0 e^{-c t} \\
& \Rightarrow \int_0^\theta d \theta=\omega_0 \int_0^t e^{-c t} d t \\
& \Rightarrow \quad \theta=\frac{\omega_0}{c}\left[1-e^{-c t}\right]
\end{aligned}
$
Hence, the answer is the option (2).
Question 3) A wedge of mass M=2kg carries two blocks A and B, as shown in figure. Block A lies on a smooth-incline surface of the wedge inclined at angle $\theta$ 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 $B=(x-1)$ towards right.
Displacement of A w.r.t wedge (on the incline) is $l$.
$\therefore$ Displacement of A in horizontal direction $=l \cos \theta+x$
The COM of the entire system suffers no displacement in horizontal direction.
$
\begin{aligned}
& \therefore \Delta x_{\mathrm{COM}}=0 \\
& \Rightarrow m_1 \Delta x_1+m_2 \Delta x_2+m_3 \Delta x_3=0 \\
& \Rightarrow M \cdot x+m(x-l)+m(l \cos \theta+x)=0 \\
& \Rightarrow x=\frac{m l(1-\cos \theta)}{M+2 m}
\end{aligned}
$
Now, putting $\mathrm{m}=1 \mathrm{~kg}, \mathrm{M}=2 \mathrm{kgI}=2 \mathrm{~m}$ and $\theta=60^{\circ}$ in (i), we get
$
\begin{aligned}
& x=\frac{1 \times 2\left(1-\cos 60^{\circ}\right)}{2+2 \times 1} \\
& x=0.25 \mathrm{~m}
\end{aligned}
$
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 ' $t_0$ ' time to stop because of uniform friction. Now if we hinge it at the corner & give same angular velocity, In $k t_0$ time it will take to stop, Find $k$.
Answer:
The torque by friction will be $\tau$ = Cμmgl and moment of inertia will be,$I=k m l^2$
$\tau$= Iα
Hence, angular acceleration will be
$\alpha=\frac{C \mu g}{k}\left(\frac{1}{l}\right)$
$\Rightarrow \alpha$ is inversely proportional to $l$
Now as it is hinged at one corner, we can assume plate of double side length
Now this is again the central axis, the value of angular acceleration will be half.
Hence, it will take $2t_0$ time to stop.
Hence, the answer is 2.
In Systems of particles and rotational motion, NCERT Class 11 Physics Chapter 6, 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
NCERT Solutions for Class 11 Physics Chapter 6: 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.
For a system of particles: $\vec{R}_{\mathrm{COM}}=\frac{\sum m_i \vec{r}_i}{\sum m_i}$
$M=\sum m_i \\$
$\vec{v}_{\mathrm{cm}}=\frac{\sum m_i \vec{v}_i}{M}$
$\vec{p}_{\mathrm{cm}}=M \vec{v}_{\mathrm{cm}}$
$\vec{a}_{\mathrm{cm}}=\frac{\vec{F}_{\mathrm{ext}}}{M}$
Average angular velocity =∆θ/∆t
Instantaneous angular velocity, ω=dθ/dt
$\begin{aligned} & I=\sum m_i r_i^2 \text { (Discrete system) } \\ & I=\int r^2 d m \text { (Continuous body) }\end{aligned}$
$\vec{\tau}=\vec{r} \times \vec{F}=r F \sin \theta$
$
\vec{L}=\vec{r} \times \vec{p}=m \vec{r} \times \vec{v}
$
For a rotating body:
$
L=I \omega
$
$\tau=I \alpha$
$K \cdot E_{\cdot \mathrm{rot}}=\frac{1}{2} I \omega^2$
$\begin{aligned} & \omega=\omega_0+\alpha t \\ & \theta=\omega_0 t+\frac{1}{2} \alpha t^2 \\ & \omega^2=\omega_0^2+2 \alpha \theta\end{aligned}$
To solve questions based on NCERT Class 11 Physics Chapter 6: the first step to solving these questions is to gain an understanding of some of the simpler concepts such as the center of mass, the torque, angular velocity, moment of inertia, and the rotational equivalents of the motion. Look keenly to determine whether the problem is a linear motion, a rotational motion or both. Employ the free-body diagrams to depict forces and torque. Use Newton laws, conservation of angular momentum and rotational laws of motion where applicable. In numerical problems make sure to convert the units properly and use of parallel or perpendicular axis theorems to compute moment of inertia. Organization and practice of various problems are important to succeed in this chapter.
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.
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.
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.
Frequently Asked Questions (FAQs)
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.
By assuming rigidity, it eliminates complications of deformation hence making the analysis easy.
Linear motion is the movement in a straight direction and rotational is the turning around an axis.
Yes it bears the same role in rotational motion as mass does in linear motion.
The most common example includes rotating wheels, ceiling fans, spinning tops, and rotating planets.
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