NCERT Class 11 Physics Physics Part-I: Chapter 4 — 1 INTRODUCTION
This chapter, "Laws of Motion," from NCERT Class 11 Physics Part-I, delves into the fundamental principles governing the motion of bodies. It begins by contrasting the description of motion with the causes of motion, introducing the concept of force as an external agency required to change the state of motion of an object. The chapter critically examines Aristotle's fallacy, which incorrectly stated that an external force is always needed to maintain motion. It then introduces Galileo's insights, particularly his work with inclined planes, which laid the groundwork for understanding inertia. The text explains that in the absence of opposing forces like friction, a body in motion would continue to move with constant velocity. This chapter is crucial for building a strong foundation in classical mechanics for CBSE students.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 11 |
| Subject | Physics |
| Book | Physics Part-I |
| Chapter | Chapter 4 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 6 minutes |
| Word count | 1075 |
Learning outcomes
- Understand the concept of force as an external agency.
- Differentiate between Aristotle's view and Galileo's concept of inertia.
- Recognize that uniform motion requires no net external force in the absence of friction.
- Appreciate the historical development of the laws of motion.
Vocabulary
| Word | Meaning |
|---|---|
| Velocity | The rate of change of displacement of an object. |
| Acceleration | The rate of change of velocity of an object. |
| Force | An external agency that can change the state of motion of a body. |
| Inertia | The tendency of an object to resist changes in its state of motion. |
| Friction | A force that opposes motion between surfaces in contact. |
| Uniform motion | Motion with constant velocity. |
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 difference between describing motion and explaining its cause? Answer: Describing motion involves quantifying velocity and acceleration, while explaining its cause involves understanding the forces acting on the body.
- State Aristotle's view on motion. Answer: Aristotle believed that an external force is required to keep a body in motion.
- What did Galileo conclude about motion on a frictionless horizontal plane? Answer: Galileo concluded that an object moving on a frictionless horizontal plane would move with constant velocity (no acceleration or retardation).
- Why does a toy car come to rest when its string is released? Answer: A toy car comes to rest due to the opposing force of friction from the floor.
Practice MCQs
Q1. Which concept is needed to describe non-uniform motion?
Explanation: Non-uniform motion involves a change in velocity, which is described by acceleration.
Q2. According to Aristotle, what is required to keep a body moving?
Explanation: Aristotle's view was that an external force is necessary to maintain motion.
Q3. Galileo's experiments with inclined planes led to the concept of:
Explanation: Galileo's work on inclined planes was foundational to the concept of inertia.
Q4. What force opposes the motion of a toy car on a floor?
Explanation: Friction between the toy car's wheels and the floor causes it to slow down and stop.
Q5. In the ideal situation described by Galileo, a ball on a frictionless horizontal plane would:
Explanation: In the absence of opposing forces, an object in motion continues in motion with constant velocity.
Frequently asked questions
What is the main question addressed in Chapter 4 of NCERT Physics Part-I?
The chapter addresses the fundamental question of what governs the motion of bodies, moving beyond just describing motion.
What was Aristotle's view on motion?
Aristotle believed that an external force is required to keep a body in motion.
How did Galileo's experiments challenge Aristotle's view?
Galileo's experiments with inclined planes suggested that in the absence of friction, a body in motion would continue to move with constant velocity.
What is inertia?
Inertia is the tendency of an object to resist changes in its state of motion.
Why do objects eventually come to rest in our everyday experience?
Objects come to rest due to opposing forces like friction and air resistance.
What is the significance of uniform motion in this chapter?
Uniform motion (constant velocity) is significant because it requires no net external force in an idealized frictionless environment, forming the basis of inertia.
Related resources
Important topics
Topics covered
NCERT Class 11 Physics — Physics Part-I — Chapter 4 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.