What is another name for the first law of motion?

• A. Law of inertia
• B. Law of momentum
• C. Law of acceleration
• D. Law of action-reaction

Answer: (A)

Inertia is the key idea here—the tendency of an object to resist changes to its motion. Because of this resistance, the first law of motion is also called the Law of Inertia. It states 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. Think of a hockey puck sliding on ice: it keeps going until friction or another force slows it down. This naming emphasizes the property of matter that resists changes in motion.

The other concepts described don’t name the law: momentum is a quantity (mass times velocity) that helps describe motion and is conserved in many situations; acceleration is how quickly velocity changes; and action-reaction refers to Newton’s third law about paired forces between interacting bodies.