NCERT Class 11 Physics Physics Part-I: Chapter 2 — 1 INTRODUCTION
This chapter, 'Motion in a Straight Line,' from NCERT Class 11 Physics Part-I, introduces the fundamental concepts of describing motion. It defines motion as a change in an object's position over time and focuses on rectilinear motion, where objects move along a straight line. The chapter develops the concepts of instantaneous velocity and speed, defining velocity as the rate of change of position with respect to time. It explains how to determine velocity from position-time graphs using calculus and numerical methods. The text also touches upon the concept of acceleration and kinematic equations for uniformly accelerated motion, laying the groundwork for understanding the causes of motion in subsequent chapters. This foundational knowledge is crucial for CBSE physics students.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 11 |
| Subject | Physics |
| Book | Physics Part-I |
| Chapter | Chapter 2 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 4 minutes |
| Word count | 798 |
Learning outcomes
- Define and describe motion, including rectilinear motion.
- Understand and calculate instantaneous velocity and speed.
- Interpret position-time graphs to determine velocity.
- Grasp the concept of acceleration in the context of motion.
- Apply kinematic equations for uniformly accelerated motion.
Vocabulary
| Word | Meaning |
|---|---|
| Motion | Change in position of an object with time. |
| Rectilinear motion | Motion of objects along a straight line. |
| Velocity | Rate of change of position with respect to time at an instant. |
| Speed | The magnitude of velocity. |
| Instantaneous velocity | Velocity at a specific instant of time. |
| Average velocity | Total displacement divided by total time interval. |
| Acceleration | Rate of change of velocity with respect to time. |
| Kinematics | Study of ways to describe motion without considering the causes. |
| Point object | An object whose size is much smaller than the distance it moves. |
| Calculus | Branch of mathematics used for finding rates of change (differentiation). |
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Practice questions
- What is the definition of motion? Answer: Motion is defined as the change in position of an object with time.
- What is rectilinear motion? Answer: Rectilinear motion is the motion of an object along a straight line.
- How is instantaneous velocity defined? Answer: Instantaneous velocity is defined as the limit of the average velocity as the time interval becomes infinitesimally small.
- What does the slope of a position-time graph represent? Answer: The slope of a position-time graph represents the velocity of the object.
Practice MCQs
Q1. Which branch of physics studies the description of motion without considering its causes?
Explanation: Kinematics is the branch of mechanics that describes the motion of points, bodies (objects), and systems of bodies without considering the forces that cause them to move.
Q2. For motion in a straight line, what is the term used for motion along a straight path?
Explanation: Rectilinear motion specifically refers to motion along a straight line.
Q3. Instantaneous velocity is the limit of average velocity as the time interval:
Explanation: Instantaneous velocity is defined using the concept of a limit where the time interval approaches zero.
Q4. The rate of change of position with respect to time at a specific instant is called:
Explanation: This is the definition of instantaneous velocity.
Q5. When can an object be treated as a point object?
Explanation: The point object approximation is valid when the size of the object is negligible compared to the distance it travels.
Frequently asked questions
What is the main focus of Chapter 2 of NCERT Physics Part-I?
Chapter 2 focuses on describing motion, specifically motion in a straight line, and introduces concepts like velocity and acceleration.
What is the difference between average velocity and instantaneous velocity?
Average velocity is the total displacement divided by the total time interval, while instantaneous velocity is the velocity at a specific moment in time, found by taking the limit of average velocity as the time interval approaches zero.
When is the 'point object' approximation useful in physics?
The point object approximation is useful when the size of the object is much smaller than the distance it travels, simplifying calculations.
What does the slope of a position-time graph represent?
The slope of a position-time graph represents the velocity of the object at that instant.
What is kinematics?
Kinematics is the study of motion without considering the forces that cause it.
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NCERT Class 11 Physics — Physics Part-I — Chapter 2 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.