What is the fundamental principle behind Newton's First Law of Motion?

The fundamental principle behind Newton's First Law of Motion is indeed that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a net external force. This law introduces the concept of inertia, which is the tendency of objects to maintain their state of motion. It emphasizes that the natural state of an object is to either remain at rest or to continue moving uniformly unless an external force disrupts that state.

This principle underlines the importance of forces in changing an object's motion. For example, if a soccer ball is sitting on the ground, it will not move until a player kicks it. Similarly, once the ball is in motion, it will continue to move forward until forces such as friction or air resistance act on it to slow it down or change its direction.

The other options involve related concepts in physics but do not directly articulate the essence of Newton's First Law. The law itself clearly defines the relationship between motion and net external forces, establishing the foundation for understanding how and why objects behave as they do in motion.