Which law describes the motion of objects when left to their own inertia?

The first law of motion, also known as Newton's first law, states that an object at rest will stay at rest, and an object in motion will continue in its motion at a constant velocity, unless acted upon by a net external force. This principle describes the concept of inertia, which is the resistance of any physical object to change in its state of motion. Inertia is a fundamental property of matter, and it highlights how objects will maintain their current state (either at rest or in uniform motion) unless an unbalanced force induces a change.

This law is significant because it lays the groundwork for understanding the behavior of objects in motion and forms a basis for the subsequent laws of motion. The other laws listed, such as the second and third laws of motion, relate to how forces affect the motion of objects but do not specifically address the state of motion when no external forces are applied. The law of gravitation pertains to the attraction between masses rather than the motion in the absence of external forces.