What causes a moving body to resist a change in its state of motion?

• A. Momentum
• B. Force
• C. Inertia
• D. Acceleration

Answer: (C)

Inertia is the tendency of matter to resist changes in its motion. A body that’s moving will keep moving in the same direction at the same speed unless a net external force acts on it. This resistance to changing motion is inertia, and it’s stronger in objects with greater mass.

That’s why inertia best explains the question: it’s the property that causes the resistance to change. Momentum describes the current motion (mass times velocity) and changes as motion changes, but it isn’t the cause of the resistance itself. Force is what can overcome inertia to alter motion, and acceleration is the rate at which velocity changes once a net force is applied.