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

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

• A. An object at rest stays at rest and an object in motion stays in motion with constant velocity unless acted on by a net external force.
• B. F_net = m a.
• C. Weight is the gravitational force on a mass.
• D. An inertial frame has fictitious forces present.

Answer: (A)

Inertia is the tendency of objects to resist changes to their motion. Newton's first law states that if no net external force acts on an object, its velocity remains constant—an object at rest stays at rest, and an object in motion continues moving in a straight line at constant speed. The statement you’re examining describes exactly that: motion remains constant unless a net external force acts. Think of a puck sliding on frictionless ice: it would keep moving with the same speed and in the same direction unless someone pushes it or there’s some force acting on it. The other ideas blur different concepts: F_net = m a is Newton's second law, linking net force to acceleration; weight is simply the gravitational force on a mass; and fictitious forces arise in non-inertial frames, not in inertial ones.

Inertia is the tendency of objects to resist changes to their motion. Newton's first law states that if no net external force acts on an object, its velocity remains constant—an object at rest stays at rest, and an object in motion continues moving in a straight line at constant speed. The statement you’re examining describes exactly that: motion remains constant unless a net external force acts.

Think of a puck sliding on frictionless ice: it would keep moving with the same speed and in the same direction unless someone pushes it or there’s some force acting on it. The other ideas blur different concepts: F_net = m a is Newton's second law, linking net force to acceleration; weight is simply the gravitational force on a mass; and fictitious forces arise in non-inertial frames, not in inertial ones.