Which statement best expresses Newton's first law of motion?

Which statement best expresses Newton's first law of motion?

• A. An object in motion will remain in motion unless acted upon by another force.
• B. Force equals mass times acceleration.
• C. For every action there is an equal and opposite reaction.
• D. A frequency with which a structure can resonate.

Answer: (A)

Inertia drives the idea that an object naturally tends to keep doing what it’s already doing. Newton's first law says an object at rest stays at rest and an object in motion stays in motion unless an external net force changes its motion. The statement describing an object in motion remaining in motion unless acted on by another force directly expresses that tendency toward constant velocity. In the real world, friction or gravity can provide that external force to alter motion, but without such a net force, velocity remains constant. The other statements describe different concepts: force equals mass times acceleration is Newton's second law; for every action there is an equal and opposite reaction is Newton's third law; and a frequency at which a structure can resonate relates to vibrational behavior.

Inertia drives the idea that an object naturally tends to keep doing what it’s already doing. Newton's first law says an object at rest stays at rest and an object in motion stays in motion unless an external net force changes its motion. The statement describing an object in motion remaining in motion unless acted on by another force directly expresses that tendency toward constant velocity. In the real world, friction or gravity can provide that external force to alter motion, but without such a net force, velocity remains constant. The other statements describe different concepts: force equals mass times acceleration is Newton's second law; for every action there is an equal and opposite reaction is Newton's third law; and a frequency at which a structure can resonate relates to vibrational behavior.