Inertia is the inherent property of a body that makes it oppose any force that would cause a change in its motion. It is a property of matter by which it remains at rest or in uniform motion in the same straight line unless acted upon by some external force. Inertia can be of two types:

• Inertia of Rest: When a body is at rest, it tends to remain at rest until an external force acts upon it.

• Inertia of Motion: When a body is in motion, it tends to remain in motion with the same speed and direction unless acted upon by an external force.

Inertia is a passive property and does not enable a body to do anything except oppose such active agents as forces and torques. A moving body keeps moving not because of its inertia but only because of the absence of a force to slow it down, change its course, or speed it up. There are two numerical measures of the inertia of a body: its mass, which governs its resistance to the action of a force, and its moment of inertia about a specified axis, which measures its resistance to the action of a torque about the same axis.