Which sentence best describes Newton's first law?

• A. An object at rest stays at rest and an object in motion stays in motion unless acted on by an unbalanced external force.
• B. For every action, there is an equal and opposite reaction.
• C. Force equals mass times acceleration.
• D. Kinetic energy depends on velocity.

Answer: (A)

Inertia is the key idea here: objects resist changes to their motion. Newton's first law says that if there’s no net external force acting on an object, it will keep doing what it’s already doing—a resting object stays at rest, and a moving object keeps moving at the same speed and in the same direction. Only a net external force can change that motion.

So the sentence that matches this idea describes motion continuing unchanged without an unbalanced force. Think of a puck sliding on a frictionless surface that would keep going straight forever, or a book sitting on a table that stays put until someone pushes it. The other statements refer to different concepts (third law pairs, F = ma, and kinetic energy), not to inertia and constant motion without net force.