Which equation expresses Newton's second law?

• A. F = ma
• B. F = m a^2
• C. p = m v
• D. W = F d

Answer: (A)

The main idea here is how a force changes motion. Newton's second law says that the net external force on an object equals its mass times its acceleration. Put simply, F = m a. This means: for a given mass, increasing the net force makes the object accelerate more; for a given force, a heavier object accelerates less. The acceleration points in the same direction as the net force, since force and acceleration are in the same direction for a single object. In SI units, a newton (N) is the force required to accelerate a 1-kilogram mass by 1 meter per second squared, so F = m a is consistent dimensionally as F (N) = m (kg) × a (m/s^2).

The other ideas described by the other equations relate to different concepts: momentum (p = m v) and work (W = F d). The remaining option isn’t consistent with the law either. So the equation that directly expresses Newton’s second law is F = m a.