What does Newton's first law of motion state?

Newton's first law of motion states that an object at rest will remain at rest, and an object in motion will continue moving at a constant velocity in a straight line unless acted upon by a net external force. This principle encapsulates the concept of inertia, which is the tendency of objects to resist changes in their state of motion. The law emphasizes the role of external forces in altering the motion of objects, highlighting that without an external influence, the current state—whether at rest or in uniform motion—will persist.

In practical terms, this means that if no external forces such as friction, gravity, or applied forces are acting on an object, it will not change its motion. This law is foundational in understanding how objects behave in the absence of forces and is essential in the study of mechanics. It sets the stage for the concepts of motion and force that are elaborated on in later laws of motion.