In all engineering construction, the fundamental components of a machine or structure are designed by keeping in mind the mechanical properties of solids. This is so that the parts are proportioned to resist the forces imposed on them. Be it the floors of a building, walls of a pressure cooker or wings of an aircraft, the right choice of materials and dimensions can make or break the parts! A sound understanding of these properties and the chapter mechanical properties of solids will help you grasp the concepts of subsequent chapters with ease and strengthen your grip over class 11 physics. So, let’s begin to learn this chapter from the class 11th physics NCERT solutions!

## Intermolecular Forces, Elasticity, and Plasticity

Intermolecular Forces – As we know, solids are made up of atoms and molecules packed close to each other. The arrangement is such that each molecule exerts a force on its neighbouring molecule. These forces are known as intermolecular forces.

Elasticity: It is the ability of a deformed body to return to its original dimensions (shape, size, and volume) when the external forces are removed. Bodies that show elasticity are called elastic bodies. Examples of perfectly elastic bodies: Quartz Fiber, Phosphor Bronze

Plasticity: It is the ability of a body to undergo permanent deformation (in shape, size, or volume) when the deforming forces are removed. Bodies that show attributes of plasticity are called plastic bodies. Examples of plastic bodies: Wax, Clay, Plasticine

Class 11 Physics Chapter 7 – System of Particles and Rotational Motion

In all engineering construction, the fundamental components of a machine or structure are designed by keeping in mind the mechanical properties of solids. This is so that the parts are proportioned to resist the forces imposed on them. Be it the floors of a building, walls of a pressure cooker or wings of an aircraft, the right choice of materials and dimensions can make or break the parts! A sound understanding of these properties and the chapter mechanical properties of solids will help you grasp the concepts of subsequent chapters with ease and strengthen your grip over class 11 physics. So, let’s begin to learn this chapter from the class 11th physics NCERT solutions!

## Intermolecular Forces, Elasticity, and Plasticity

Intermolecular Forces – As we know, solids are made up of atoms and molecules packed close to each other. The arrangement is such that each molecule exerts a force on its neighbouring molecule. These forces are known as intermolecular forces.

Elasticity: It is the ability of a deformed body to return to its original dimensions (shape, size, and volume) when the external forces are removed. Bodies that show elasticity are called elastic bodies. Examples of perfectly elastic bodies: Quartz Fiber, Phosphor Bronze

Plasticity: It is the ability of a body to undergo permanent deformation (in shape, size, or volume) when the deforming forces are removed. Bodies that show attributes of plasticity are called plastic bodies. Examples of plastic bodies: Wax, Clay, Plasticine

Class 11 Physics Chapter 7 – System of Particles and Rotational Motion

## Stress, Strain, and Hooke’s Law

Stress

In mechanical properties of solids, stress is defined as the restoring force per unit area. Hence, Stress in SI units is N/m2 or Pascal (Pa). Types of Stresses in Solids:

• Normal Stress/ Longitudinal Stress- It is defined as the type of stress that occurs when an axial force is applied perpendicular to the surface. Eg. Pressure. Normal stresses can be further divided into:
• Shearing Stress/ Tangential Stress– It is the type of stress that occurs when the restoring force acts parallel or tangentially to the surface of the body. Eg. Cutting a piece of paper with scissors.

Class 11 Physics Chapter 3 – Motion in a Straight Line

Strain

Strain is defined as the measure of deformation or elongation of an object when acted upon by an external force. Since it deals with the relative change in length or shape of an object, it is a dimensionless quantity. Types of strains are:

• Longitudinal Strain/Tensile Strain- As the name suggests, longitudinal strain occurs when the deforming forces produce a change in the body’s length
• Shearing Strain: The strain produced when tangential stress causes an angle tilt in the body
• Volumetric strain- The strain produced when the deforming forces cause only a change in the volume of a body

Hooke’s Law

As per the chapter, Mechanical Properties of Solid, Hooke’s law expresses the relationship between the two main mechanical properties of solids- stress and strain. It states that, within an elastic range, the strain within an object (consider a spring in this case) is proportionate to the stress applied to it. Therefore, the ratio of stress to strain is given by a constant. This constant is called the modulus of elasticity. Hence, As strain is just a number, SI units of the modulus of elasticity are the same as stress (N/m2 or Pascal (Pa)).

## Mechanical Properties of Solids: Stress-Strain Curve

As mentioned in the chapter mechanical properties of solids, a stress-strain curve is a reliable way to determine and evaluate the mechanical properties of solids. It can help you understand the load-bearing capacity or tensile strength of a material. Let’s take metals for this example.

A– Shown as a linear curve. This is the region where the material obeys Hooke’s law, i.e. The material regains its original position when the load is removed

B– Also known as the elastic limit or yield point. The stress at this point is called the yield strength of the material

B to C– This curve denotes the region of plastic deformation

C– Point C shows the ultimate strength or tensile strength of the material

D– Any additional strain beyond this point may lead to permanent fracture of the material

Modern Physics

## Important Ratios Defining Mechanical Properties of Solids

Young’s Modulus– Within the elastic limits of a thin wire or rod, it is defined as the ratio of longitudinal stress to longitudinal strain.

Bulk Modulus- Measures the ability of a substance to withstand compression from all sides. It is defined as the ratio of normal stress to volumetric strain

Shear Modulus/ Modulus of Rigidity- According to the mechanical properties of solids chapter, it is defined as the ratio of shear stress to shear strain

Poisson’s Ratio- It is defined as the ratio between lateral strain (change in diameter) and longitudinal strain (change in length)

Basic Physics Formulas & Notes for Competitive Exams

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