Newton’s First Law.

A body continue to be in its state of rest or of uniform motion along a straight line, unless it is acted upon by some external force to change the state.

(1) If no net force acts on a body, then the velocity of the body remains constant.

(2) Newton’s first law defines inertia so is rightly called the law of inertia.

Inertia are of three types :

a)Inertia of rest

b) Inertia of motion

c) Inertia of direction

Inertia of rest : It is the inability of a body to change by itself, its state of rest. This means a body at rest remains at rest and cannot start moving by its own.

e.g. A person who is standing freely in bus, thrown backward, when bus starts suddenly.

Inertia of motion : It is the inability of a body to change itself its state of uniform motion i.e., a body in uniform motion can neither accelerate nor retard by its own.

e.g. When a bus or train stops suddenly, a passenger sitting inside tends to fall forward. This is because the lower part of his body comes to rest with the bus or train but the upper part tends to continue its motion due to inertia of motion.

Inertia of direction :It is the inability of a body to change by itself direction of motion.

e.g. When a stone tied to one end of a string is whirled and the string breaks suddenly, the stone flies off along the tangent to the circle.