# Mechanical Properties of Solids

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## Mechanical Properties of Solids Class 11 Notes

In this article, students will get to learn and fetch mechanical properties of solids class 11 notes. We are going to discuss the properties that solids have such as elastic behaviour of solids, stress and strain, stress-strain curve, Hooke’s law and elastic moduli.

### What are the Mechanical Properties of Solids?

Mechanical properties of solids elaborates the characteristics such as the resistance to deformation and their strength. Strength is the ability of an object to withstand the applied stress, to what extent can it bear the stress. Resistance to deformation is how resistant any object is to the change of shape. If the resistance to deformation is less, the object can easily change its shape and the vice versa.

Therefore, some of the mechanical properties of solids include:

1. Elasticity: When we stretch an object, it changes its shape and when we leave, it regains its shape. Or we can say it is the property to regain the original shape once the external force is removed. Example: Spring

1. Plasticity: When an object changes its shape and never comes back to its original shape even when external force is removed. It is the property of permanent deformation. Example: Plastic materials.

1. Ductility: When an object can be pulled in thin sheets, wires or plates, it has the ductile properties. It is the property of being drawn into thin wires/sheets/plates. Example: Gold or Silver

1. Strength: The ability to withstand applied stress without failure. Many categories of objects have higher strength than others.

There are various other properties but in this chapter of class 11 Physics mechanical properties of solids, we will mainly focus on the elasticity of solids.

### Stress and Different Types of Stress

It is the restoring force that develops on an object in the opposite direction; it is measured per unit area. For example, when a rubber ball is applied by an external force with our hands to compress it, at the same time the ball develops an opposite force that restrains it; however, both the forces are equal in magnitude. This restoring force developed by the object or ball is called stress.

Stress = Force/Area

The S.I. unit of Stress is N/m square or Pascal (Pa)

Different Types of Stress are:

1. Longitudinal Stress: Longitude means length-wise; therefore, it can be defined as the restoring force per unit area when the force applied is normal to the cross-sectional area of the cylindrical body. There is change in the length of the object taking place. Example, when a cylindrical rubber object is tied with a heavy object, there will be longitudinal stress acting upon and the change in the length of the object takes place.

Longitudinal stress is divided into two sub-categories:

• Tensile stress: In the above example, it can be said that tensile stress develops when force is applied to stretch the cylinder.

• Compressive stress: When force is applied to compress the object.

2. Tangential or Shearing Stress

It is the restoring force per unit area when the force applied is parallel to the cross-sectional area of the body. There is a relative displacement occurring between the opposite faces of the body.

3. Hydraulic Stress

It is the restoring force per unit area when the force is applied by a fluid like water on the body or object. Suppose, a ball made of rubber (which can be compressed) is dipped inside a river or sea, there is a force acting on the ball from all directions due to the pressure of the water. It results in the minor contraction of the ball. It is an example of hydraulic stress that you can include in the notes of mechanical properties of solids.

### Strain and Different Kinds of Strain

It is a measure of the deformation that can represent the displacement between particles in the body relative to a reference length.

Strain is dimensionless quantity. If a rubber object is stretched from both the sides, the change in length represents the strain.

Different Types of Strain are:

1. Longitudinal Strain: It is the change in length to the original length of the body due to the applied longitudinal stress. It is a change in length divided by the original length.

1. Shearing Strain: It is the measure of the relative displacement of the opposite faces of the body due to the shearing stress. Shearing strain can be represented by tan Θ.

1. Volume Strain: It is the ratio of change in volume to the original volume as a result of the hydraulic stress. It is the change in volume divided by the initial or original volume.

### Hooke’s Law

It is named after the scientist Robert Hooke. Hooke’s Law states that stress developed is directly proportional to the strain produced in an object, within elastic limit (if the object is elastic material). An object that can come back to the original shape is its elasticity. Therefore, hooke’s law applies to elastic objects. It doesn’t apply to the plasticity property of solids.

It can be, therefore, represented as Stress = k * Strain

Where, k is the modulus of elasticity

### Stress-Strain Curve

A curve drawn between stress and strain is called the stress-strain curve. When stress and stress are drawn along the y-axis and x-axis respectively, a linear graph is formed in the ideal situation of Hooke’s law. However, when actual experiments are drawn, a curve is formed  known as the stress-strain curve as shown below.