The property of an object by virtue of which it cannot change its state of rest or of uniform motion along a straight line its own, is called inertia.

Inertia is a measure of mass of a body. Greater the mass of a body greater will be its inertia or vice-versa.

1. Inertia is of three types:

1. Inertia of Rest When a bus or train starts to move suddenly, the passengers sitting in it falls backward due to inertia of rest.
2. Inertia of Motion When a moving bus or train stops suddenly, the passengers sitting in it jerks in forward direction due to inertia of motion.
3. Inertia of Direction We can protect yourself from rain by an umbrella because rain drops cannot change its direction its own due to inertia of direction.

1. FORCE

Force is a push or pull which changes or tries to change the state of rest, the state of uniform motion, size or shape of a body.

Its SI unit is newton (N) and its dimensional formula is [MLT-2].

2. Forces can be categorized into two types:

1. Contact Forces Frictional force, tensional force, spring force, normal force, etc. are the contact forces.
2. Action at a Distance Forces Electrostatic force, gravitational force, magnetic force, etc are action at a distance forces.

• Impulsive Force

A force which acts on body for a short interval of time, and produces a large change in momentum is called an impulsive force.

• Linear Momentum

The total amount of motion present in a body is called its momentum. Linear momentum of a

body is equal to the product of its mass and velocity. It is denoted by p.

Linear momentum p = mu.

Its S1 unit is kg-m/s and dimensional formula is [MLT-1].

It is a vector quantity and its direction is in the direction of velocity of the body.

• Impulse

The product of impulsive force and time for which it acts is called impulse.

Impulse = Force * Time = Change in momentum

Its S1 unit is newton-second or kg-m/s and its dimension is [MLT-1].

It is a vector quantity and its direction is in the direction of force.