Inertia is the natural tendency of an object to resist any change in its state of motion. This means an object at rest tends to stay at rest, and an object in motion tends to continue moving at the same velocity in a straight line unless acted upon by an external force. This concept is a fundamental principle of classical physics and is described by Isaac Newton's first law of motion, often called the principle of inertia

. Inertia is not an active force but a passive property of matter that opposes changes in motion. It does not cause motion but resists changes to it. For example, a moving object keeps moving not because of its inertia but because no external force is acting to stop or alter its motion

. The amount of inertia an object has depends primarily on its mass. The greater the mass, the greater the inertia, meaning it is harder to change the motion of a more massive object. There is also a rotational equivalent called the moment of inertia, which measures resistance to changes in rotational motion about an axis

. In summary:

• Inertia is the resistance to change in motion or rest.
• It is described by Newton's first law: objects maintain their state unless acted on by an external force.
• It is a passive property, not a force itself.
• Mass is the measure of inertia; more mass means more inertia.
• Moment of inertia applies to rotational motion.

This principle explains everyday phenomena, such as why passengers lurch forward when a car suddenly stops-their bodies resist the change in motion due to inertia