When any force is applied to the solids, they undergo deformation. This deformation depends upon the amount of force and the resistance power of a solid. However, the solid again regains its initial form after removal of the force as it as elasticity.

For example, when you stretch a rubber band, it deforms, but it gets back its original shape and size after removing the force. Thus, we can say it possesses elasticity. However, when we apply a force and deform an iron rod. It doesn’t return to its original shape or size. Thus, it doesn’t possess elasticity.

The amount of force put on the object per unit area is called stress. The strain is the change in length which has to be divided by the original length per unit area. Moreover, the stress and strain are directly proportional to one another.

Types of Stress-

• Tensile Stress- When there is a change in length of an object under the action of specific force per unit area, it is called tensile stress.
• Compressive Stress- When there is a change in the width of an object under the action of specific force per unit area, it is called compressive stress.

Types of Strain-

• Tensile Stress- When there is a change in length of an object under the action of specific force per unit area, it is called a tensile strain.
• Compressive Stress- When there is a change in the width of an object under the action of specific force per unit area, it is called compressive strain.

Robert Hooke, an English scientist, gave Hook’s law in the 19th century. This law states that change in size due to deformation is directly proportional to the force of deformation over the object.

The mathematical expression for Hooke’s law is

F = –k.x,

Where F= force, x= extension of length, and k is the spring constant.

Hooke’s law has its importance in various fields of engineering and science. It is the only way of understanding the elastic behaviour of materials.

The graphical presentation of stress and strain values is the Stress-strain curve. These curves can explain other properties also. These properties are the Young modulus, the yield strength, and ultimate tensile strength.

This graph also helps in studying the behaviour of materials as well.

Elastic Moduli of any solid is the quantity of the measure of the ratio of force to the resultant deformation due to that force.

There are three types of Elastic Moduli-

• Young’s modulus- Young’s modulus (Y) is the ratio of the given compressive stress (σ) to the longitudinal strain (ε).

Y = σ/ε

Where σ =compressive stress and ε =longitudinal strain

• Shear modulus- Shear Modulus is the ratio of the present shearing stress to the shearing strain. 

G= = FL/A Δx. 

Where (FL) =shear stress and (A Δx) = shear strain.

• Bulk modulus- Bulk modulus is defined as the ratio of hydraulic stress to the present hydraulic strain when a body submerges in a fluid.

P = −κ(V − V₀)/V₀, where P= pressure, k= constant, and V= volume.

Question 1- What is Elastic Moduli?

Answer 1- The Elastic modulus is the ratio of stress to the strain.

There are three types of Elastic Moduli-

• Young’s modulus (Y) - Young’s modulus is the ratio of compressive stress (σ) to the longitudinal strain (ε). 

Y = σ/ε, where σ is the compressive stress, and ε is the longitudinal strain.

• Shear modulus- It is the ratio of the shearing stress to the shearing strain. 

G= = FL/A Δx, where (FL) is the shear stress and (A Δx) is the shear strain.

• Bulk modulus is the ratio of hydraulic stress to the corresponding hydraulic strain when a body submerges in a fluid.

P = −κ (V − V0)/V0, where P= pressure, k= constant, and V= volume.

Question 2- What is Hooke’s law?

Answer 2- This law states that a change in size due to deformation is directly proportional to the force present over the object. Robert Hooke, an English scientist, gave this law in the 19th century.

Mathematical expression for Hooke’s law-

F = –k.x,

Where F is the force, x is the length extension, and k is the spring constant.

Hook’s law is very useful in determining the elastic behaviour of various substances. Moreover, this is the only method for describing the elastic behaviour. It is beneficial in various fields of science and engineering.

Question 3- What is intermolecular force?

Answer 3- Intermolecular force is the force present between molecules that cause characteristic features of that particular substance. Moreover, these forces held the molecules of the substance together.

As solids have the greatest intermolecular force due to which solids are rigid and their molecules are held tightly. Liquids have a slightly greater intermolecular force than gases. Thus, gases possess the least of it; therefore, the molecules of gases roam freely.

The intermolecular force depends on various factors. Some of these important factors are-

• Dipole-Dipole Interactions. 
• Ion-Dipole Interactions.
• Dipole Induced Dipole Interaction.
• Ion Induced Dipole Interactions.
• Dispersion Forces or London Forces.

Question 4- Tips to score well in Class 11 Physics, Chapter 9? 

Answer 4- To get a better score in Class 11 Physics, Chapter 9, you must read the chapter thoroughly. Read it from the NCERT books for the best understanding. Firstly, get aware of all the basics of the chapter. The basics act as the building blocks for any further understanding of the chapter.  After the complete knowledge of the basics, go through the other topics as well. Now, practice as many questions as you can get. Practising the questions will master the topic.

Question 5- What are the practical applications of the Mechanical Properties of Solids Class 11?

Answer 5- The mechanical properties of any substance define its usefulness in various fields. Moreover, it is useful in classifying the materials based on their properties, such as strength, hardness, or ductility. It is an essential part of the engineering field. This clears us of various phenomena logically occurring around us. The principles of Mechanical Properties of Solids are also useful in the Euler-Bernoulli equation.