Newton's first law states that an object in motion will remain in motion until acted upon by:

• A. A stronger force
• B. An opposing force
• C. An internal energy change
• D. An unbalanced external force

Answer: (D)

Inertia means an object in motion tends to keep moving at the same speed and direction unless something else acts on it. What actually changes that motion is a net external force—the sum of all outside forces acting on the object. When that net external force is not zero, the object accelerates in the direction of the net force, changing its velocity. That’s why the correct statement is an unbalanced external force: it produces acceleration and alters the motion. If external forces balance, motion remains constant; nothing changes. An opposing force by itself doesn’t guarantee a change unless it creates a net unbalanced force. Internal energy changes don’t dictate motion in this law.