What is Newton's first law of motion?

Newton's first law of motion describes the fundamental principle of inertia, stating that an object will maintain its state of rest or uniformly straight-line motion unless a net external force acts on it. This means that if no forces are applied to an object, it will continue to either not move or to move at a constant velocity. This law highlights the concept of inertia, which is the tendency of an object to resist changes in its motion. In practical terms, it explains why seatbelts are important in cars; when a car suddenly stops, a passenger will continue moving at the same speed unless stopped by an external force, like the seatbelt.

The other options pertain to different principles of physics. The second option relates to the law of conservation of energy; the third discusses kinetic energy formula, showing the relationship between velocity and mass of moving objects; and the fourth statement introduces the concept of force and its effect on motion, which aligns more with Newton's second law. Each of these concepts is essential, but they do not define what Newton's first law states about the behavior of objects at rest and in motion.