It states that sum of all the forces acting on a moving body is equal to its inertial force.

Σ F-Ma=0

where

M→mass of the body

a→acceleration of the body

Examples:

(i) The acceleration of a lift is a result of combination of tension in the cables and weight it carries.

(ii) A body sliding down an inclined plane has a net acceleration due to component of its weight and frictional force.