NCERT Class 11 Physics Physics Part-II: Chapter 2 — 1 INTRODUCTION
This chapter introduces the mechanical properties of liquids and gases, collectively known as fluids. Unlike solids, fluids lack a definite shape and offer minimal resistance to shear stress, distinguishing them fundamentally. The chapter defines pressure as the force acting per unit area, emphasizing its scalar nature and SI unit, Pascal (Pa). It explains that fluids at rest exert normal forces on surfaces due to Newton's third law and the inability of fluids to sustain shear stress. Density, defined as mass per unit volume, is also introduced as a crucial property, with a distinction made between the low compressibility of liquids and the high compressibility of gases. The concept of relative density is also presented. Understanding these properties is vital as fluids are ubiquitous in nature and biological systems.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 11 |
| Subject | Physics |
| Book | Physics Part-II |
| Chapter | Chapter 2 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 5 minutes |
| Word count | 976 |
Learning outcomes
- Differentiate between solids and fluids based on their mechanical properties.
- Define and calculate pressure exerted by fluids.
- Understand the scalar nature of pressure and its SI unit.
- Explain why fluids at rest exert normal forces on surfaces.
- Define density and relative density and their units.
- Recognize the difference in compressibility between liquids and gases.
Vocabulary
| Word | Meaning |
|---|---|
| Fluids | Liquids and gases, substances that can flow. |
| Pressure | The force acting per unit area, perpendicular to the surface. |
| Pascal (Pa) | The SI unit of pressure (N m-2). |
| Density | Mass per unit volume of a substance (ρ = m/V). |
| Relative density | The ratio of the density of a substance to the density of water at 4°C. |
| Compressibility | The measure of the relative change in volume in response to a change in pressure. |
| Shear stress | Stress that causes deformation by sliding of layers. |
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Practice questions
- What is the main difference between solids and fluids? Answer: Fluids can flow and have no definite shape of their own, unlike solids.
- Define pressure and state its SI unit. Answer: Pressure is the force acting per unit area. Its SI unit is Pascal (Pa).
- Why is the force exerted by a fluid at rest always normal to the surface? Answer: If there were a parallel component of force, it would cause the fluid to flow, which contradicts the condition that the fluid is at rest.
- What is density? Answer: Density is the mass of a substance per unit volume.
- Give an example of a common unit of pressure other than Pascal. Answer: Atmosphere (atm) is a common unit of pressure.
Practice MCQs
Q1. Which of the following is a key property distinguishing fluids from solids?
Explanation: Fluids are defined by their ability to flow and lack of a definite shape, unlike solids which resist flow and maintain shape.
Q2. What is the SI unit of pressure?
Explanation: The SI unit of pressure is the Pascal (Pa), named after Blaise Pascal, equivalent to one Newton per square meter (N m-2).
Q3. Pressure is defined as:
Explanation: Pressure is the magnitude of the normal force acting per unit area.
Q4. Which statement about liquids and gases is correct regarding compressibility?
Explanation: Liquids and solids have much lower compressibility compared to gases, meaning their volume changes less significantly with pressure changes.
Q5. The density of a fluid is defined as:
Explanation: Density (ρ) is defined as the mass (m) of a substance divided by its volume (V), i.e., ρ = m/V.
Frequently asked questions
What are fluids according to this chapter?
Fluids are substances that can flow and do not have a definite shape of their own. This category includes liquids and gases.
How is pressure different from force?
Force is a push or pull, while pressure is the force acting per unit area. Pressure takes into account both the magnitude of the force and the area over which it is distributed.
What is the SI unit of pressure and who is it named after?
The SI unit of pressure is the Pascal (Pa), named in honour of the French scientist Blaise Pascal.
Why do fluids at rest exert force perpendicular to a surface?
If a fluid at rest exerted a force parallel to a surface, it would cause the fluid to flow, which contradicts the condition of being at rest. Therefore, the force must be normal (perpendicular).
What is density and how is it calculated?
Density is a measure of mass per unit volume. It is calculated by dividing the mass of a substance by its volume (ρ = m/V).
What is relative density?
Relative density is the ratio of the density of a substance to the density of water at 4°C. It is a dimensionless quantity.
Are liquids or gases more compressible?
Gases are much more compressible than liquids. The change in volume of liquids due to pressure changes is relatively small.
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NCERT Class 11 Physics — Physics Part-II — Chapter 2 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.