What does Newton's first law of motion state?

Newton's first law of motion, also known as the law of inertia, asserts that an object will remain at rest or in uniform motion in a straight line unless acted upon by a net external force. This principle highlights the concept of inertia, which is the tendency of an object to resist changes in its state of motion.

In this context, if no external force is applied, a stationary object won't start moving, and a moving object won't change its velocity or trajectory. This foundational concept is crucial in understanding how forces interact with objects, laying the groundwork for subsequent laws of motion. It emphasizes that motion is not inherent to an object; rather, it requires a force to initiate or alter that motion.

The other choices focus on different principles of physics: free fall and acceleration relate to gravitational forces, force and acceleration are part of Newton's second law, and the conservation of energy highlights the fundamental principles of thermodynamics. However, these concepts are distinct from the definition of Newton's first law.