Which statement best describes Newton's First Law?

Which statement best describes Newton's First Law?

• A. An object accelerates when a net force is applied
• B. An object at rest stays at rest unless acted on by a net external force
• C. Force equals mass times acceleration
• D. The momentum of a closed system is conserved

Answer: (B)

Inertia—the tendency to maintain whatever motion or rest the object currently has—underlies Newton’s First Law. An object at rest stays at rest and an object in motion continues moving with the same velocity unless a net external force acts to change that state. So, if there’s no net force, the velocity remains constant; if there is a net force, the velocity changes (the object accelerates). The statement you chose captures this idea directly: it describes the condition where no net external force is acting to change the object’s state. The other ideas describe what happens when forces are present (acceleration under a net force, F = ma) or a different principle (momentum conservation) and don’t convey the same fundamental tendency to resist changes in motion.

Inertia—the tendency to maintain whatever motion or rest the object currently has—underlies Newton’s First Law. An object at rest stays at rest and an object in motion continues moving with the same velocity unless a net external force acts to change that state. So, if there’s no net force, the velocity remains constant; if there is a net force, the velocity changes (the object accelerates).

The statement you chose captures this idea directly: it describes the condition where no net external force is acting to change the object’s state. The other ideas describe what happens when forces are present (acceleration under a net force, F = ma) or a different principle (momentum conservation) and don’t convey the same fundamental tendency to resist changes in motion.