## INTERMOLECULAR FORCE

In a solid, atoms and molecules are arranged in such a way that each molecule is acted upon by the forces due to the neighbouring molecules. These forces are known as intermolecular forces.

## ELASTICITY

- The property of the body to regain its original configuration (length, volume or shape) when the deforming forces are removed, is called elasticity.
- The change in the shape or size of a body when external forces act on it is determined by the forces between its atoms or molecules. These short range atomic forces are called elastic forces.

## PERFECTLY ELASTIC BODY

A body which regains its original configuration immediately and completely after the removal of deforming force from it, is called a perfectly elastic body. Quartz and phospher bronze are examples of nearly perfectly elastic bodies.

## PLASTICITY

The inability of a body to return to its original size and shape even on removal of the deforming force is called plasticity and such a body is called a plastic body.

## STRESS

Stress is defined as the ratio of the internal force F, produced when the substance is deformed, to the area A over which this force acts. In equilibrium, this force is equal in magnitude to the externally applied force.

### TYPES OF STRESS

(i) * Normal stress: *It is defined as the restoring force per unit area perpendicular to the surface of the body. Normal stress is of two types: tensile stress and compressive stress.

(ii)

__When the elastic restoring force or deforming force acts parallel to the surface area, the stress is called tangential stress.__

*Tangential stress:*## STRAIN

It is defined as the ratio of the change in size or shape to the original size or shape.
It has no dimensions, it is just a number.

### TYPES OF STRAIN

### (i) Longitudinal strain:

If the deforming force produces a change in length alone, the strain produced in the body is called longitudinal strain or tensile strain. It is given as:

### (ii) Volumetric strain:

If the deforming force produces a change in volume alone, the strain produced in the body is called volumetric strain. It is given as:

### (iii) Shear strain:

The maximum stress to which the body can regain its original status on the removal of the deforming force is called elastic limit.

## HOOKE'S LAW

Hooke’s law states that, within elastic limits, the ratio of stress to the corresponding strain produced is a constant. This constant is called the modulus of elasticity. Thus

## STRESS STRAIN CURVE

Stress strain curves are useful to understand the tensile strength of a given material.

## YOUNG'S MODULUS

For a solid, in the form of a wire or a thin rod, Young’s modulus of elasticity within elastic limit is defined as the ratio of longitudinal stress to longitudinal strain. It is given as:

## BULK MODULUS

- Within the elastic limit the bulk modulus is defined as the ratio of longitudinal stress and volumetric strain.
- It is given as:

- – ve indicates that the volume variation and pressure variation always negate each other.
- Reciprocal of bulk modulus is commonly referred to as the “compressibility”.
- It is defined as the fractional change in volume per unit change in pressure.

## SHEAR MODULUS OR MODULUS OF RIGIDITY

It is defined as the ratio of the tangential stress to the shear strain.
Modulus of rigidity is given by

## POISSON'S RATIO

- The ratio of change in diameter (ΔD) to the original diameter (D) is called lateral strain.
- The ratio of change in length (Δl) to the original length (l) is called longitudinal strain.
- The ratio of lateral strain to the longitudinal strain is called Poisson’s ratio.

## ELASTIC FATIGUE

It is the property of an elastic body by virtue of which its behaviour becomes less elastic under the action of repeated alternating deforming forces.

## RELATIONS BETWEEN ELASTIC MODULI

- For isotropic materials (i.e., materials having the same properties in all directions), only two of the three elastic constants are independent.
- For example, Young’s modulus can be expressed in terms of the bulk and shear moduli.

## BREAKING STRESS

The ultimate tensile strength of a material is the stress required to break a wire or a rod by pulling on it. The breaking stress of the material is the maximum stress which a material can withstand. Beyond this point breakage occurs.