NCERT Class 11 Physics Physics Part-I: Chapter 6 — 1 INTRODUCTION
This chapter, "Systems of Particles and Rotational Motion," from NCERT Class 11 Physics Part-I, moves beyond the study of single particles to explore the motion of extended bodies. It introduces the concept of a system of particles and the crucial idea of the centre of mass, which helps in understanding the overall motion of such bodies. The chapter defines a rigid body as an idealised object with a fixed shape, acknowledging that real bodies deform but can often be treated as rigid for practical purposes. It distinguishes between translational motion, where all particles move with the same velocity, and rotational motion about a fixed axis, exemplified by objects like fans and wheels. This foundational understanding is essential for analysing complex mechanical systems in physics.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 11 |
| Subject | Physics |
| Book | Physics Part-I |
| Chapter | Chapter 6 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 5 minutes |
| Word count | 823 |
Learning outcomes
- Understand the limitations of the particle model for extended bodies.
- Define and identify the centre of mass of a system of particles.
- Differentiate between translational and rotational motion.
- Recognize the concept of a rigid body and its applicability.
- Identify examples of translational and rotational motion in real-world objects.
Vocabulary
| Word | Meaning |
|---|---|
| Particle | An idealized point mass having no size. |
| Extended body | A body of finite size. |
| System of particles | An assembly of multiple particles. |
| Centre of mass | A point representing the mean position of the matter in a body or system. |
| Rigid body | A body with a perfectly definite and unchanging shape. |
| Translational motion | Motion where all particles of a body have the same velocity at any instant. |
| Rotational motion | Motion about a fixed axis where the body rotates. |
| Axis of rotation | The fixed line about which a rigid body rotates. |
The complete chapter text is read in the official NCERT PDF viewer below (streamed from ncert.nic.in). This page provides NCERT Help study material — summary, vocabulary, practice questions, and FAQs — not a full reproduction of the textbook.
Read chapter online
This PDF is loaded from the official NCERT website (ncert.nic.in). Use the page buttons below to read — download is disabled on NCERT Help.
Read page by page below. PDF is streamed from the official NCERT website — no download button on this page.
Practice questions
- What is the idealised model used in earlier chapters for motion? Answer: The idealised model used was that of a particle, represented as a point mass having no size.
- When is the idealised model of a particle inadequate for describing motion? Answer: The particle model is inadequate when dealing with the motion of extended bodies of finite size.
- Define a rigid body. Answer: A rigid body is ideally a body with a perfectly definite and unchanging shape, where the distances between all pairs of particles remain constant.
- What is the characteristic of pure translational motion? Answer: In pure translational motion, at any instant of time, all particles of the body have the same velocity.
- Give examples of bodies that can be treated as rigid. Answer: Examples include wheels, tops, steel beams, molecules, and planets, where deformations are often negligible.
Practice MCQs
Q1. What is an idealized representation of motion in earlier chapters?
Explanation: Earlier chapters primarily considered the motion of a single particle, which is ideally represented as a point mass with no size.
Q2. Which concept is key to understanding the motion of extended bodies?
Explanation: The centre of mass of a system of particles is a key concept for understanding the motion of extended bodies as a whole.
Q3. According to the definition, a rigid body has:
Explanation: Ideally, a rigid body is defined as having a perfectly definite and unchanging shape, meaning distances between particles do not change.
Q4. In pure translational motion, all particles of a body:
Explanation: The defining characteristic of pure translational motion is that all particles within the body share the same velocity at any given instant.
Q5. Which of the following is an example of rotation about a fixed axis?
Explanation: A ceiling fan rotates about a fixed central axis, making it a prime example of rotational motion about a fixed axis.
Frequently asked questions
What is the difference between a particle and an extended body in physics?
A particle is an idealized point mass with no size, used for simplifying motion analysis. An extended body has finite size and its motion might require considering its internal structure or treating it as a system of particles.
What is the significance of the centre of mass?
The centre of mass is a crucial concept that represents the average position of mass in a system. Its motion can often describe the overall motion of an extended body, simplifying complex analyses.
Can real bodies be perfectly rigid?
No, ideally a rigid body has a fixed shape. Real bodies deform under forces, but they can often be approximated as rigid if the deformations are negligible for the problem at hand.
What distinguishes translational motion from rotational motion?
In translational motion, all parts of the body move with the same velocity. In rotational motion, the body moves around a fixed axis, with different points moving at different velocities depending on their distance from the axis.
What is an axis of rotation?
The axis of rotation is the fixed line about which a rigid body turns or spins.
Give examples of rotational motion about a fixed axis.
Common examples include a ceiling fan, a potter's wheel, a spinning top, and a giant wheel in a fair.
Related resources
Important topics
Topics covered
NCERT Class 11 Physics — Physics Part-I — Chapter 6 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.