What is the key concept described by Newton’s First Law of Motion?

Newton's First Law of Motion, often referred to as the law of inertia, states that an object at rest will remain at rest, and a moving object will continue to move at a constant velocity in a straight line unless acted upon by a net external force. This concept highlights the natural tendency of objects to resist changes to their state of motion.

In practical terms, it means that if there are no forces acting on an object — for instance, if it is in a vacuum where no friction or air resistance affects it — then it will stay in its current state, whether that be stationary or moving uniformly. This principle is fundamental in understanding the behaviors of objects in motion and helps frame concepts in both classical mechanics and everyday experiences.

The other options refer to different principles of physics. The concept of needing force to maintain motion relates more closely to Newton’s Second Law, while the third option describes Newton's Third Law of Motion. The last option ties into inertia and mass but does not fully capture the essence of the First Law, which centers specifically on the conditions of motion and rest.