Which statement expresses Newton's Second Law of Motion?

• A. For every action there is an equal and opposite reaction.
• B. Energy equals mass times speed of light squared.
• C. F = m a.
• D. F = m v.

Answer: (C)

Newton's Second Law describes how motion changes in response to a force. It says the net external force acting on an object equals the rate of change of its momentum, which for everyday situations becomes F = m a: the net force is proportional to the object's mass times its acceleration, and it points in the same direction as the acceleration. In practice, doubling the net force while keeping mass the same doubles the acceleration; increasing the mass with the same force reduces the acceleration. This law links cause (force) to effect (motion change) and explains why heavier objects need more force to achieve the same acceleration. The other statements refer to different ideas: equal and opposite forces are from Newton's Third Law, energy-mass–speed of light is Einstein’s relation, and F = m v would incorrectly tie force to velocity.