Which statement reflects Newton's First Law?

Which statement reflects Newton's First Law?

• A. The force equals mass times acceleration
• B. For every action there is an equal and opposite reaction
• C. An object at rest stays at rest and an object in motion stays in motion unless acted on by a net external force
• D. Energy cannot be created or destroyed

Answer: (C)

Inertia drives Newton's First Law: objects resist changes to their motion. An object at rest stays at rest, and an object moving with a constant velocity keeps moving that way unless a net external force acts on it. The key idea is that motion only changes when something external nudges it—gravity, friction, thrust, or drag, for example, sum up to a net force that alters speed or direction. If those forces cancel out, the object's motion remains constant. That statement captures inertia and the idea of a net external force causing a change in motion, which is exactly what Newton's First Law describes. The other statements describe different principles: force equals mass times acceleration is Newton's Second Law, which explains how and why motion changes when a net force is present; the equal-and-opposite reaction is Newton's Third Law, about action-reaction pairs; and energy cannot be created or destroyed is the conservation of energy.

Inertia drives Newton's First Law: objects resist changes to their motion. An object at rest stays at rest, and an object moving with a constant velocity keeps moving that way unless a net external force acts on it. The key idea is that motion only changes when something external nudges it—gravity, friction, thrust, or drag, for example, sum up to a net force that alters speed or direction. If those forces cancel out, the object's motion remains constant.

That statement captures inertia and the idea of a net external force causing a change in motion, which is exactly what Newton's First Law describes. The other statements describe different principles: force equals mass times acceleration is Newton's Second Law, which explains how and why motion changes when a net force is present; the equal-and-opposite reaction is Newton's Third Law, about action-reaction pairs; and energy cannot be created or destroyed is the conservation of energy.