default_background
✕

Find the best tutors and institutes for Class 11 Tuition

Find Best Class 11 Tuition

Please select a Category.

Please select a Locality.

No matching category found.

No matching Locality found.

Q7.29(a):

Explain why friction is necessary to make the disc in Fig. 7.41 roll in the direction indicated. (a) Give the direction of frictional force at B, and the sense of frictional torque, before perfect rolling begins.

Solution :

1. Theoretical Foundation of Rolling Motion

For a disc to undergo rolling motion, it must satisfy two conditions simultaneously: linear motion of its center of mass ($v_{cm}$) and rotational motion about its center of mass ($\omega$). Pure rolling is defined by the constraint $v_{cm} = R\omega$. When a disc is placed on a surface with an initial velocity $v_0$ but without sufficient initial rotation, the contact point $B$ experiences a relative sliding velocity $v_B = v_{cm} - R\omega$ [Per the principle of relative motion].

2. Role of Friction in Rolling

Friction is the external agency that provides the torque necessary to change the angular velocity of the disc. Without friction, the disc would continue to slide indefinitely at constant $v_{cm}$ and $\omega = 0$. Friction at the point of contact $B$ acts to oppose the direction of sliding, thereby decelerating the linear motion and accelerating the angular motion [Per Newton’s Second Law for Rotation: $\vec{\tau} = I\vec{\alpha}$].

3. Direction of Frictional Force and Torque

Given the disc is moving forward with $v_{cm}$ and initially has no rotation or insufficient rotation ($\omega$ is small), the contact point $B$ has a velocity relative to the ground pointing forward. Therefore, the kinetic frictional force $f_k$ acts in the direction opposite to the sliding velocity to oppose it [Per the Law of Kinetic Friction].

  • Direction of Frictional Force at B: Backward (opposite to the direction of $v_{cm}$).
  • Sense of Frictional Torque: Since the force acts at the bottom point $B$ directed backward, the torque $\vec{\tau} = \vec{r} \times \vec{f}$ relative to the center of mass $C$ is directed into the page. According to the right-hand rule, this produces an angular acceleration $\alpha$ that induces a clockwise rotation, increasing the magnitude of $\omega$ [Per Right-Hand Thumb Rule].

4. Visual Representation of Dynamics

v_cm f_k B ω

5. Conclusion

The frictional force $f_k$ acts in the direction opposite to the center-of-mass velocity to reduce $v_{cm}$ and simultaneously creates a torque $\tau = f_k R$ about the center of mass. This torque provides the angular acceleration $\alpha = \frac{\tau}{I} = \frac{f_k R}{MR^2/2} = \frac{2f_k}{MR}$, which increases the angular velocity $\omega$ until the condition of pure rolling ($v = R\omega$) is achieved.

Final Solution: The frictional force at point B acts in the backward direction. The frictional torque is directed such that it produces a clockwise angular acceleration to initiate or increase the rotation of the disc.


More Questions from Class 11 Physics System of Particles and Rotational Motion EXERCISES


CBSE Solutions for Class 11 Physics System of Particles and Rotational Motion


Chapters in CBSE - Class 11 Physics


Top Tutors who teach System of Particles and Rotational Motion

(16)
Top Tutor
locationImg Crossings Republik, Ghaziabad
experience 20 yrs of Exp students 9 students
rsIcon 500 per hour
20 years of experience accounts for 11th,12th,b.com.,m.com,bba,mba,cpt,ca,cs
Online Classes Online Classes
homeTutions Student's Home
regularClasses Tutor's home

I am an experienced, qualified teacher and tutor with over 20 years of experience in teaching accounts and income tax across different boards including CBSE, ICSE, IGCSE and State Board for different classes 11th,12th,b.com,m.com,bba,mba,cpt,ca,ipcc,cs . Passionate about solving student's problems over the years I have helped thousands of students overcome their fear of accounts and income tax. So far, I have worked as a Teacher with radiant academy, desired tutors, expert tutors, tutor nation, career makers.

Ankit Mavi

Best accounts teacher. Very friendly. Supportive sir. Thanks sir for your help.

Skills: Economics , Accounts and more
Also teaches: BBI Tuition, Class 10 Tuition and more

(3)
Top Tutor
locationImg Sector 38, Chandigarh
experience 8 yrs of Exp students 1 student
greenClock 10 hrs of classes completed
Online Classes Online Classes

i am a teacher....I am giving home/online tuition since last 8years..I have a degree in masters of biotechnology...My key skills are best in competition as well as theoratical classes...I worked as biology project head in various institute as well as in various schools for preparing kids for NEET and for other medical entrance exams.

Skills: Biology

Kalaiselvi Class 11 Tuition trainer in Chennai
(28)
Top Tutor
locationImg Anna Nagar Mullai Nagar, Chennai
experience 9 yrs of Exp students 6 students
greenClock 420 hrs of classes completed
rsIcon 600 per hour
Online Mathematics tutor with 8 years experience(Online Classes for 10th to 12th)
Online Classes Online Classes

Hi, I'm Kalaiselvi, an M.Sc. Mathematics graduate with 9 years of experience in one-on-one online Maths tutoring. I specialize in teaching Classes 10, 11, and 12 for CBSE and Tamil Nadu State Board. My classes are conducted in both English and Tamil (bilingual), making concepts easy to understand. I focus on concept clarity, problem-solving, regular practice, and exam preparation, with personalized attention for every student.

Jaishna

Great experience with Kalaiselvi Ma'am, her knowledge and skill on the subject are amazing. Thank you for your support and guidance.

Skills: Mathematics

(10)
Top Tutor
locationImg Nigdi, Pune
experience 6 yrs of Exp
greenClock 1 hrs of classes completed
rsIcon 500 per hour
Junior Scientist in Biomedical Field
Online Classes Online Classes
homeTutions Student's Home
regularClasses Tutor's home

I have excellent teaching experience of 7 years with best results with my previous student's from CBSE , STATE and ICSE BOARD'S . Preferably i focus on students personal interest, level of understanding and study methodology. I am here to give best of me to seek best out from my students. I have given seminar on 'Students changing Virtue towards study , Need if time??' . Passionate to guide students towards there destiny

Skills: Biology
Also teaches: NEET-UG Coaching, Class 10 Tuition and more

(51)
Top Tutor
locationImg SCB Nagar, Kothagudem
experience 20 yrs of Exp students 2 students
greenClock 2893 hrs of classes completed
rsIcon 500 per hour
Mathematics Facilitator 22 Years experience
Online Classes Online Classes

5 years of experience in Kiit International school for 12 standard CBSE CLASSES and 4 years of experience in NARAYANA e Techno in Rayachoti Kadapa.which is equal to +1 syllabus

Skills: Mathematics
Also teaches: Math Olympiad, Class 7 Tuition and more

Other Subjects in CBSE - Class 11

Worksheet Icon

Download free CBSE - Class 11 Physics System of Particles and Rotational Motion EXERCISES worksheets

Download Now

Find Best Class 11 Tuition ?

Find Now »