Which principle explains the behavior of objects in motion unless acted upon by an external force?

The principle that explains the behavior of objects in motion unless acted upon by an external force is Newton's First Law. This law, often referred to as the law of inertia, states that an object at rest will remain at rest, and an object in motion will continue moving at a constant velocity in a straight line unless acted upon by a net external force. This means that without any external influences, such as friction or applied forces, the motion of an object remains unchanged.

Understanding this concept is crucial in physics because it lays the foundation for how forces and motion interact. For example, when you slide a book across a table, it eventually comes to a stop due to the external force of friction acting upon it, illustrating that something must intervene to alter its state of motion.

This principle is distinct from Newton's Second Law, which focuses on how the velocity of an object changes when it is subjected to an external force, and the Law of Universal Gravitation, which describes the gravitational attraction between massive bodies. Conservation of Energy involves the transformation of energy from one form to another, but does not specifically address the state of motion unless a force is acting. Thus, Newton's First Law is the foundational explanation surrounding the motion of objects relative to external forces.