What principle states that an object at rest will stay at rest and an object in motion will stay in motion unless an external force acts upon it?

The principle that describes that an object at rest will remain at rest and an object in motion will continue to move at a constant velocity unless acted upon by an external force is known as the Law of Inertia. This fundamental concept, first formulated by Sir Isaac Newton, establishes a key understanding of how forces interact with the physical state of objects.

Inertia refers to the tendency of an object to resist changes in its state of motion. This means that without an external influence, like a push or pull, a stationary object will not start to move, and a moving object will not slow down or stop. This principle is essential in understanding various concepts of motion and is foundational in fields such as physics and biomechanics, making it relevant for fitness instructors who need to understand how forces affect body movements during exercise.

The other options do not accurately describe this principle. The Law of Motion generally refers to Newton's three laws of motion, while the Law of Limbs does not correspond to any recognized scientific law, and the Law of Gravity specifically pertains to the attraction between masses rather than states of motion.