What is Newton's first law?

Newton's first law of motion, often referred to as the law of inertia, essentially states that an object will remain at rest or continue to move in a straight line at a constant velocity unless influenced by an external force. This principle asserts that there is a tendency of objects to resist changes in their state of motion.

The choice indicating that "bodies at rest stay at rest, and bodies in motion stay in motion," encapsulates the essence of this law by describing the inertia of objects. An object at rest remains in that state until a net force acts upon it, just as an object in motion will not change its velocity unless a force intervenes. This characteristic of matter is fundamental to understanding motion and is foundational in physics, serving as one of the key principles that explains how forces act on objects.

In contrast to other choices, which also reflect elements of this law, they might emphasize different aspects or contexts of the principle. Newton's first law focuses on the inherent properties of motion and rest related to external forces. Therefore, the correct choice provides a clearer, more comprehensive understanding of this foundational concept of classical mechanics.