Final answer:
Newton's first law of motion, also known as the law of inertia, describes that a body remains at rest or in constant velocity motion unless influenced by an external force. It is intimately related to the concept of inertia, which is dependent on the object's mass.
Step-by-step explanation:
Newton's First Law of Motion
Newton's first law of motion, often referred to as the law of inertia, is a fundamental principle that describes the motion of objects. It states that a body at rest remains at rest, and a body in motion remains in motion at a constant velocity unless acted on by a net external force. This concept is critical in understanding the behavior of objects in both rest and motion.
Inertia is the inherent property of an object to resist any change in its state of motion. The amount of inertia an object has is directly related to its mass; the greater an object's mass, the more inertia it possesses, making it harder to change its state of motion.
Understanding this law is essential as it applies to everyday phenomena and is a cornerstone of classical mechanics. In the context of your question, the option that accurately fills the blank in the statement "A theory of physics - that a body in motion stays in motion" would be 'asserts', referring specifically to Newton's First Law of Motion.