What is described by Newton's first law of motion?

Newton's first law of motion states that an object at rest will remain at rest, and an object in motion will continue in motion at a constant velocity, unless acted upon by a net external force. This principle emphasizes the concept of inertia, which is the tendency of an object to resist changes in its state of motion.

The correct answer highlights how an object will maintain its current state—whether that be remaining still or continuing to move in a straight line—if no external forces interfere. This foundational law of motion is essential for understanding how forces interact with objects and is a building block for Newton's other laws, which further develop the concepts of motion and force.

In contrast, the other choices relate to different principles in Newton's laws of motion. The second option describes the third law, which addresses action and reaction forces. The third option articulates the second law, detailing the relationship between force, mass, and acceleration. The last option misconstrues motion as it suggests that objects inherently speed up over time, which contradicts the first law by implying some form of force consistently acting on the object.