Final answer:
Newton's First Law, or the law of inertia, states that an object at rest stays at rest and an object in motion maintains its constant velocity unless acted on by a net external force.
Step-by-step explanation:
The law of motion you're asking about is Newton's First Law, often referred to as the law of inertia. According to Newton's first law, an object at rest will stay at rest, and an object in motion will stay in motion at a constant velocity, which means it will move at a constant speed in a straight line, unless acted upon by an unbalanced, or net external force. This fundamental principle of physics helps us understand the movement and behavior of objects.