Inertia is a property of matter that describes its resistance to a change in motion or state of rest. It is a fundamental concept in classical mechanics and is closely related to mass. The concept of inertia was first described by Galileo, who observed that objects at rest tend to stay at rest, and objects in motion tend to stay in motion with a constant velocity, unless acted upon by an external force. This observation is known as Newton's first law of motion, which states that an object at rest will remain at rest, and an object in motion will remain in motion with a constant velocity, unless acted upon by an unbalanced force.
Inertia is directly proportional to an object's mass, meaning that the more massive an object is, the greater its inertia. It is also dependent on the object's velocity, with objects moving at higher velocities having greater inertia than those moving at lower velocities. The concept of inertia has many practical applications, such as in the design of vehicles and structures, the study of planetary motion, and the analysis of impact and collision dynamics.