In order for an object to move, which of the following must act on it?

• A. Mass
• B. Acceleration
• C. Speed
• D. Force

Answer: (D)

Motion occurs when a net external force acts on an object. According to F = ma, that force produces acceleration, changing the object's velocity. Mass describes how resistant an object is to changing its motion (inertia), so more force is needed to achieve the same acceleration with a heavier object. Speed just tells you how fast something is moving; it’s a property of motion, not something that causes motion. If there’s no net force, an object would keep moving at whatever velocity it already has, or stay at rest if it’s not moving to begin with. So the force acting on an object is what starts and changes its motion.