Which statement describes Newton's second law of motion?

Which statement describes Newton's second law of motion?

• A. An object will not change its motion unless a force acts upon it
• B. Every action has an equal and opposite reaction
• C. Energy equals mass times speed of light squared
• D. Force is equal to mass x acceleration

Answer: (D)

Newton's second law describes how forces change motion. The key idea is that the acceleration of an object is proportional to the net force acting on it and inversely proportional to its mass, and it points in the same direction as the net force. When mass is constant, this is written as F = m a: increasing the force makes the object accelerate more, while increasing the mass makes the acceleration smaller. More generally, the net force equals the rate of change of momentum, so if mass changes over time, F = dp/dt still governs how velocity evolves. Think of pushing a light cart versus a heavy cart with the same push—the lighter cart speeds up more quickly, while the heavier one accelerates less. The other statements describe different ideas: one is inertia, which says motion tends to continue without a force; another is the action–reaction pair of Newton’s third law; and the last is the relativity mass–energy equivalence.

Newton's second law describes how forces change motion. The key idea is that the acceleration of an object is proportional to the net force acting on it and inversely proportional to its mass, and it points in the same direction as the net force. When mass is constant, this is written as F = m a: increasing the force makes the object accelerate more, while increasing the mass makes the acceleration smaller. More generally, the net force equals the rate of change of momentum, so if mass changes over time, F = dp/dt still governs how velocity evolves. Think of pushing a light cart versus a heavy cart with the same push—the lighter cart speeds up more quickly, while the heavier one accelerates less. The other statements describe different ideas: one is inertia, which says motion tends to continue without a force; another is the action–reaction pair of Newton’s third law; and the last is the relativity mass–energy equivalence.