What is Newton's First Law of Motion?

Newton's three laws of motion: (1) FIRST LAW (Inertia): An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force. Objects 'want' to keep doing what they're doing; inertia is the resistance to change in motion. Mass measures inertia — more massive objects are harder to start, stop, or change direction. Example: a book on a table stays still until you push it; a hockey puck on frictionless ice would slide forever. (2) SECOND LAW (F = ma): The net force on an object equals its mass times its acceleration. F = ma. Force is measured in Newtons (kg·m/s²). Acceleration is the change in velocity over time. Mass is constant for an object. Example: more force on the same mass = more acceleration; same force on more mass = less acceleration. (3) THIRD LAW (Action-Reaction): For every action, there is an equal and opposite reaction. When object A exerts force on object B, B exerts equal but opposite force on A. Example: when you push against a wall, the wall pushes back with equal force; a rocket pushes hot gas out the back, the gas pushes the rocket forward. Newton's laws apply to non-relativistic, non-quantum scales — everyday physics. Einstein's relativity modifies them at near-light speeds; quantum mechanics modifies them at atomic scales.