What describes the tendency of a mass to resist changes in its state of motion?

The tendency of a mass to resist changes in its state of motion is known as inertia. This concept comes from Newton's First Law of Motion, which states that an object at rest will stay at rest, and an object in motion will continue in motion with the same speed and in the same direction unless acted upon by an external force. Inertia is intrinsically related to mass; the greater the mass of an object, the greater its inertia, meaning it will resist changes in motion more than an object with less mass. Therefore, inertia fully describes how a mass behaves in the face of forces applied to it. This fundamental principle underlines much of classical mechanics and is crucial for understanding motion and forces in physics.