A moving object accelerates when acted upon by a net external force. This relationship is described by which law?

• A. Newton's First Law of Motion
• B. Newton's Third Law of Motion
• C. Newton's Second Law of Motion
• D. Law of Conservation of Momentum

Answer: (C)

Acceleration occurs when a net external force acts on a moving object. This is captured by Newton’s Second Law, which links force, mass, and acceleration: the acceleration is in the direction of the net force and has magnitude equal to the net force divided by the mass (F = ma). In other words, a larger net force makes the object speed up or slow down more, while a heavier mass slows the rate of acceleration for the same force. If there’s no net external force, the velocity remains constant.

The other ideas describe different situations: a body at rest or in motion will continue that state only if the net force is zero (inertia); forces during interactions come in equal and opposite pairs (action-reaction); and momentum changes in interactions within a closed system without external influences (conservation of momentum). These are related concepts, but the direct link between a net external force and a change in motion is the second law.