Which law states that an object in motion stays in motion unless acted upon by an external force?

Newton's First Law, also called the Law of Inertia, states that an object will remain at rest or in uniform motion unless acted upon by an external force. Mathematically, F = ma shows that when force F is zero, acceleration a is also zero, meaning the object's velocity remains constant. This principle explains why passengers lurch forward when a car suddenly stops; their bodies resist the change in motion due to inertia. The law is fundamental in mechanics, explaining motion in the absence of external forces such as friction and air resistance.