The UPSC mains GS-3 paper syllabus has various subjects like Economics, Science and Technology. Science and technology in UPSC is mostly concerned with current Affairs. but it is necessary to understand the basics of science.


  • What is Elasticity?
  • Hooke's Law


When an external force is applied to a rigid body there is a change in its length, volume (or) shape. When external forces are removed the body tends to regain its original shape and size. Such a property of a body by virtue of which a body tends to regain its original shape (or) size when external forces are removed is called elasticity.

What is Stress?

When the body is deformed by the application of external forces, forces within the body are brought into play. Elastic bodies regain their original shape due to internal restoring forces. The internal forces and external forces are opposite in direction.  If a force F is applied uniformly over a surface of area A then the stress is defined as the force per unit area.

Stress = Force/Area

S.I unit for stress is Nm-2

Types of Stress

There are three types of stress

  • Longitudinal stress
  • Volume stress or Bulk Stress
  • Tangential stress (or) shear stress

Longitudinal Stress

When the stress is normal to the surface area of the body and there is a change in the length of the body it is known as longitudinal stress.

Again it is classified into two types

Tensile stress

  • Compressive stress. - Tensile stress: When longitudinal stress produced due to an increase in the length of the object is known as tensile stress.
  • Compressive stress: Longitudinal stress produced due to the decrease in length of the object is known as compressive stress.
  • Volume Stress or Bulk Stress If equal normal forces applied to the body causes a change in volume of the body, the stress is called volume stress.
  • Tangential Stress  - When the stress is tangential (or) parallel to the surface of the body is known as Tangential (or) Shear stress. Due to this shape of body changes (or) gets twisted

What is a Strain?

A body under stress gets deformed. The fractional change in the dimension of a body produced by the external stress acting on is called strain. The ratio of charge of any dimension to its original dimension is called strain. Since strain is the ratio of two identical physical quantities, it is just a number. It has no unit and dimension.

Strain also classified into three types

  • Longitudinal strain
  • Volume strain
  • Shearing strain or tangential strain

Hooke’s Law

If deformation is small, the stress in a body is proportional to the corresponding strain, this fact is known as Hooke’s law.

Within elastic limit, Stress & strain