Which of the following is Newton's First Law of Motion, generally termed the Law of Inertia?

• A. Every body persists in its state of rest, or of motion in a straight line, unless acted upon by some outside force
• B. For every action there is an equal and opposite reaction
• C. Velocity is constant in a frictionless environment
• D. Mass times acceleration equals force

Answer: (A)

Newton's First Law, the Law of Inertia, says that an object resists changes to its motion: a body at rest stays at rest and a body in motion continues moving in a straight line with constant speed unless a net external force acts on it. This captures the idea that motion, once established, does not spontaneously change; only something pushing or pulling on the object can alter its velocity or direction. In everyday life we see this whenever friction or gravity apply a force that slows, stops, or deflects motion, but in a hypothetical scenario without such forces, a moving object would keep moving along the same straight path at the same speed. The other statements describe different ideas: one is the action–reaction principle, another is a specific implication of no external forces but not the law itself, and the last is the equation of motion from the Second Law, which relates force, mass, and acceleration.