What is Newton's first law of motion?

What is Newton's first law of motion?

• A. The strength of a force is measured in Newtons.
• B. Every action has an equivalent and opposite reaction.
• C. An object's inertia will allow it to keep going as long as there are no external forces.
• D. An object will accelerate in the direction of the force applied.

Answer: (D)

Inertia governs motion: objects resist changes to how they're moving, so without a net external force, an object will keep moving in a straight line at constant speed (or stay at rest). That’s the essence of Newton’s first law. The best choice conveys this idea: an object’s inertia causes it to keep moving as long as there are no external forces acting on it. That captures both staying at rest and continuing in straight, unaccelerated motion when no net force is present. The other statements point to different ideas: one mentions the unit of force (a separate concept), another is Newton’s third law about action-reaction pairs, and another describes acceleration proportional to force (Newton’s second law).

Inertia governs motion: objects resist changes to how they're moving, so without a net external force, an object will keep moving in a straight line at constant speed (or stay at rest). That’s the essence of Newton’s first law.

The best choice conveys this idea: an object’s inertia causes it to keep moving as long as there are no external forces acting on it. That captures both staying at rest and continuing in straight, unaccelerated motion when no net force is present.

The other statements point to different ideas: one mentions the unit of force (a separate concept), another is Newton’s third law about action-reaction pairs, and another describes acceleration proportional to force (Newton’s second law).