NCERT Class 11 Physics Physics Part-II: Chapter 1 — 1 INTRODUCTION
This chapter, "Mechanical Properties of Solids," from NCERT Class 11 Physics, introduces the concept that solid bodies are not perfectly rigid and can be deformed by external forces. It defines elasticity as the property of regaining original shape and size after the removal of deforming force, and plasticity as the tendency to remain permanently deformed. The chapter explains stress as the restoring force per unit area and strain as the measure of deformation. It details different types of stress (tensile, compressive, shearing, hydraulic) and strain (longitudinal, shearing, volumetric). The importance of elastic behavior in engineering design, such as for buildings, bridges, and automobiles, is highlighted, setting the stage for understanding how materials respond to applied loads.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 11 |
| Subject | Physics |
| Book | Physics Part-II |
| Chapter | Chapter 1 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 5 minutes |
| Word count | 990 |
Learning outcomes
- Define and differentiate between elasticity and plasticity.
- Understand the concepts of stress and strain.
- Identify different types of stress and strain (tensile, compressive, shearing, longitudinal, volumetric).
- Recognize the importance of elastic properties in engineering applications.
- Explain the basic principles behind the deformation of solid bodies.
Vocabulary
| Word | Meaning |
|---|---|
| Rigid body | A body with a definite shape and size that does not deform under applied force. |
| Elasticity | The property of a body to regain its original size and shape after the removal of deforming force. |
| Plasticity | The property of a body to get permanently deformed and not regain its original shape after the removal of deforming force. |
| Deforming force | An external force applied to change the shape or size of a body. |
| Restoring force | The internal force developed in a body that opposes the deformation and tends to bring it back to its original state. |
| Stress | The restoring force per unit area developed in a body. |
| Strain | The ratio of change in dimension to the original dimension of the body. |
| Tensile stress | Stress due to stretching or elongation of a body. |
| Compressive stress | Stress due to compression or shortening of a body. |
| Shearing stress | Stress due to tangential forces causing relative displacement between opposite faces. |
| Longitudinal strain | Strain due to change in length (elongation or compression). |
| Shearing strain | Strain caused by shearing stress, measured as the ratio of relative displacement to length. |
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Practice questions
- What is the difference between elasticity and plasticity? Answer: Elasticity is the ability to regain original shape after deformation, while plasticity is the tendency to remain permanently deformed.
- Define stress and strain. Answer: Stress is the restoring force per unit area, and strain is the ratio of change in dimension to the original dimension.
- Name the different types of stress mentioned in the chapter. Answer: Tensile stress, compressive stress, shearing stress, and hydraulic stress.
- What is longitudinal strain? Answer: Longitudinal strain is the change in length divided by the original length of the body.
- Why is the knowledge of elastic properties important in engineering design? Answer: It helps engineers design structures and components that can withstand applied loads without failure, ensuring safety and efficiency.
Practice MCQs
Q1. A rigid body is defined as a body that:
Explanation: A rigid body is ideally considered one that does not change its shape or size when external forces are applied. Real bodies, however, do deform.
Q2. The property of a body to regain its original shape after the removal of deforming force is called:
Explanation: Elasticity is the characteristic of materials that allows them to return to their original shape and size once the deforming force is removed.
Q3. Stress is defined as:
Explanation: Stress is the internal restoring force that develops within a body per unit area when it is subjected to an external deforming force.
Q4. Which of the following is a type of strain?
Explanation: Shearing strain is a measure of deformation caused by shearing stress, representing the relative displacement of parallel layers.
Q5. If a force is applied normal to the cross-sectional area of a body, the stress developed is:
Explanation: When a force is applied normal to the cross-section, it leads to either stretching (tensile stress) or compression (compressive stress).
Q6. Putty and mud are examples of substances that exhibit:
Explanation: Putty and mud get permanently deformed and do not regain their original shape, which are characteristics of plastic materials.
Frequently asked questions
What is the fundamental difference between a rigid body and a real solid?
A rigid body is an idealization that does not deform under force, whereas real solid bodies can be stretched, compressed, or bent when external forces are applied.
What is the SI unit of stress?
The SI unit of stress is Pascal (Pa), which is equivalent to Newton per square meter (N/m²).
How is shearing strain defined?
Shearing strain is defined as the ratio of the relative displacement of the faces (∆x) to the length of the body (L) perpendicular to the displacement, or approximately as the angle of deformation (θ).
When is stress called tensile stress, and when is it compressive stress?
Stress is tensile when the applied forces cause elongation, and compressive when they cause shortening of the body.
What is hydraulic stress?
Hydraulic stress occurs when a solid body is subjected to uniform pressure from all sides by a fluid, causing it to compress without changing its shape.
Why is understanding elasticity crucial for engineers?
Engineers need to understand elastic properties to design structures like buildings, bridges, and vehicles that can withstand loads safely and efficiently without permanent deformation or failure.
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NCERT Class 11 Physics — Physics Part-II — Chapter 1 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.