Newton's Three Laws of Motion

Inertia, F=ma, and action-reaction pairs

Newton's First Law (Inertia): An object at rest stays at rest; an object in motion stays in motion at constant velocity, unless acted upon by a net external force. Inertia is the resistance to change in motion. Newton's Second Law (F=ma): The net force on an object equals its mass times its acceleration. A larger force or smaller mass means greater acceleration. Newton's Third Law (Action-Reaction): When object A exerts a force on object B, object B simultaneously exerts an equal and opposite force on object A. Note: these forces act on DIFFERENT objects — they never cancel. Rocket engines work by Newton's Third Law: exhaust gas pushed backward → rocket pushed forward.