Which law describes that an object will only change its motion when a force is applied?

The principle that describes how an object will only change its motion when a force is applied is encapsulated in Newton's First Law of Motion. This law states that an object at rest will remain at rest, and an object in motion will continue in its motion at a constant velocity in a straight line unless acted upon by a net external force.

This fundamental concept emphasizes the idea of inertia, which is the property of an object to resist changes in its state of motion. In practical terms, it means that forces must be exerted to alter the speed or direction of an object's movement. Without the application of a force, whether it be friction, a push, or other interactions, the object's motion remains unchanged.

This law lays the groundwork for understanding how forces interact with objects, setting the stage for more complex dynamics described in Newton's other laws. Newton's Second Law relates to the effects of forces and acceleration, while the Third Law deals with the action-reaction principle. The Law of Universal Gravitation, on the other hand, specifically addresses the gravitational force between masses, rather than the broader concept of motion and inertia.