NCERT Class 11 Physics Physics Part-I: Chapter 5 — 1 INTRODUCTION
This chapter, "Work, Energy and Power," from NCERT Class 11 Physics Part-I, introduces fundamental concepts in physics. It clarifies the precise scientific definitions of 'work,' 'energy,' and 'power,' contrasting them with their everyday usage. The chapter emphasizes that while common language uses these terms loosely, physics assigns them specific meanings. It begins by developing the mathematical prerequisite of the scalar product (dot product) of two vectors, explaining its definition, properties, and geometric interpretation. This scalar product is crucial for understanding work, which is defined as the product of force and displacement. The chapter aims to build a solid understanding of these interconnected physical quantities, laying the groundwork for further study in mechanics and energy transformations within the CBSE curriculum.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 11 |
| Subject | Physics |
| Book | Physics Part-I |
| Chapter | Chapter 5 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 6 minutes |
| Word count | 1022 |
Learning outcomes
- Understand the precise physical definitions of work, energy, and power.
- Differentiate between the everyday and physics meanings of these terms.
- Learn the mathematical concept of the scalar product (dot product) of two vectors.
- Apply the scalar product to understand the definition of work.
- Grasp the geometric interpretation of the scalar product.
Vocabulary
| Word | Meaning |
|---|---|
| Work | In physics, a precise measure of force applied over a distance. |
| Energy | The capacity to do work. |
| Power | The rate at which work is done or energy is transferred. |
| Scalar Product | A product of two vectors that results in a scalar quantity (also known as dot product). |
| Vector | A quantity having direction as well as magnitude. |
| Projection | The component of one vector along the direction of another vector. |
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Practice questions
- What is the scalar product of two vectors A and B? Answer: A.B = AB cos θ, where θ is the angle between A and B.
- If two vectors are perpendicular, what is their scalar product? Answer: Their scalar product is zero.
- Give an example of a physical quantity that is a vector. Answer: Displacement, velocity, acceleration, force.
- What does the scalar product represent geometrically? Answer: It is the product of the magnitude of one vector and the component of the other vector along the first vector.
Practice MCQs
Q1. In physics, 'work' is defined precisely as:
Explanation: In physics, work is defined as the product of the force applied to an object and the distance it moves in the direction of the force.
Q2. Which of the following is a mathematical operation that results in a scalar from two vectors?
Explanation: The scalar product, also known as the dot product, of two vectors yields a scalar quantity.
Q3. If the angle between two vectors A and B is 90 degrees, their scalar product A.B is:
Explanation: Since cos(90°) = 0, the scalar product A.B = AB cos(90°) = 0.
Q4. The term 'energy' in physics is related to:
Explanation: Energy is defined as the capacity to do work. Therefore, energy and work are closely related concepts in physics.
Q5. The scalar product A.B is equal to B.A. This property is called:
Explanation: The property A.B = B.A demonstrates that the scalar product follows the commutative law.
Frequently asked questions
What is the main difference between the everyday and physics definitions of 'work'?
In everyday language, 'work' can refer to any mental or physical effort. In physics, 'work' has a precise definition involving force applied over a distance.
How is 'energy' defined in physics?
Energy in physics is defined as the capacity to do work.
What is the scalar product of two vectors?
The scalar product (or dot product) of two vectors A and B is defined as A.B = AB cos θ, where θ is the angle between them. It results in a scalar quantity.
What does the scalar product represent geometrically?
Geometrically, the scalar product A.B is the product of the magnitude of vector A and the component of vector B along A (or vice versa).
When is the scalar product of two non-zero vectors equal to zero?
The scalar product of two non-zero vectors is zero when the vectors are perpendicular to each other (i.e., the angle between them is 90 degrees).
What are the properties of the scalar product?
The scalar product is commutative (A.B = B.A) and obeys the distributive law (A.(B+C) = A.B + A.C).
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NCERT Class 11 Physics — Physics Part-I — Chapter 5 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.