In physics, inertia is described as:

Inertia refers to the property of matter that causes it to resist changes in its state of motion. When an object is at rest, it tends to remain at rest unless acted upon by an external force. Conversely, if it is in motion, it will continue to move in a straight line at constant speed unless a force causes it to change its velocity. This principle is fundamental to Newton's first law of motion, which states that an object will not change its motion unless a net external force is applied.

This understanding helps explain various phenomena in everyday life, such as why a passenger lunges forward in a car that suddenly stops; the passenger's body wants to maintain its motion due to its inertia. The concept of inertia emphasizes the relationship between mass and motion, highlighting that the greater the mass of an object, the greater its inertia and, thus, the greater the force needed to change its state of motion.