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

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

Answer: (D)

Inertia is the tendency of an object to resist changes to its motion. Newton's first law says that if there is no net external force acting on an object, its motion won't change: a stationary object stays at rest, and an object moving in a straight line at constant speed continues to do so. Only a net external force can cause a change in that motion (speed up, slow down, or turn).

So the best statement captures this idea: without a net external force, an object's state of motion remains the same. When a net external force does act, the motion changes according to F = ma.

For context, imagine a book at rest on a smooth table: without a push, it stays put because horizontal forces balance to zero. A puck sliding on frictionless ice keeps moving in a straight line at the same speed; if friction or a push acts (a net external force), its motion changes. The other options describe different concepts: one confuses motion with force independence, another is about energy conservation, and another is about action-reaction pairs.