What principle states that an object in motion remains in motion unless acted on by a net force?

The principle that states an object in motion remains in motion unless acted on by a net force is known as Newton's first law of motion. This law describes the concept of inertia, which is an object's tendency to resist changes in its state of motion. Essentially, if no external force is applied to an object, it will continue to move at a constant velocity in a straight line or remain at rest if it is not already in motion. This principle is foundational in understanding motion and forces in physics, as it highlights the relationship between force and the behavior of objects. It underlines that the presence of a net force is necessary to change the velocity of an object, either by changing its speed or direction.

In contrast, the other laws defined by Newton address different aspects of motion and forces. The second law relates to how the acceleration of an object is dependent on the net external force and its mass, while the third law states that for every action, there is an equal and opposite reaction, emphasizing the interaction between two objects rather than the behavior of a single object in motion. Inertia, while closely related to the first law, specifically describes the property of matter that opposes changes to its motion rather than the broader principle that governs motion itself.