Inertia is a property of matter by which it resists any change in its state of rest or uniform motion along a straight line unless acted upon by some external force. It is one of the fundamental concepts in physics and is described by Newtons first law of motion, also known as the law of inertia. The law of inertia states that an object at rest will remain at rest, and an object in motion will continue in motion with a constant velocity, unless acted upon by an external force.

Inertia can be divided into three types:

• Inertia of rest: The tendency of a body to oppose any change in its state of rest is known as inertia of rest. For example, when a bus suddenly starts moving forward, the passengers in the bus fall backward because the lower part of their bodies being in contact with the floor of the bus come in motion along with the bus, while the upper part of their bodies remain at rest due to inertia of rest.

• Inertia of motion: The tendency of a body to oppose any change in its state of uniform motion is known as inertia of motion. For example, the passengers fall forward when a fast-moving bus stops suddenly because the lower part of their bodies comes to rest as soon as the bus stops, but the upper parts of their bodies continue to move due to inertia of motion.

• Inertia of direction: The tendency of a body to resist any change in its direction of motion is known as inertia of direction.

The mass of an object is the measure of its inertia, and the greater the mass, the greater the inertia.