What is the first law of motion as defined by Sir Isaac Newton?

The first law of motion, often referred to as the law of inertia, states that a body at rest remains at rest and a body in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an external force. This principle highlights the concept of inertia, which is the tendency of an object to resist changes in its state of motion.

This law underscores the importance of external forces in influencing the motion of objects, emphasizing that without such forces, an object's state will remain constant—whether that state is at rest or in uniform motion. This foundational concept is vital in physics, as it lays the groundwork for understanding how forces interact with objects to change their motion.

The other options address aspects of motion and force but do not encapsulate the true essence of the first law. One option describes the second law, focusing on the relationship between force and momentum, while another aligns with the third law, relating to action and reaction forces. The fourth option suggests a more environmental approach to motion change, which does not align with Newton's clear statement regarding the persistence of motion unless acted upon.