What is Newton's First Law of Motion commonly called?

• A. The law of gravity
• B. The law of momentum
• C. The law of inertia
• D. The law of action-reaction

Answer: (C)

Inertia—the tendency of an object to resist changes in its motion. Newton's First Law says that an object at rest stays at rest and an object in motion continues moving in a straight line at constant speed unless a net external force acts on it. This is why it’s commonly called the law of inertia: it describes the natural resistance to changes in motion. Gravity describes a force pulling masses together, momentum is a quantity that changes when forces act, and action-reaction is Newton’s Third Law about equal and opposite forces acting on different objects.