What fundamental property keeps an object at rest or in uniform motion unless acted upon by an external force?

• A. Mass
• B. Inertia
• C. Weight
• D. Density

Answer: (B)

Inertia is the tendency of any object to resist a change in its motion. Because of inertia, something at rest stays at rest and something in motion keeps moving in a straight line at a constant speed unless a net external force acts to alter its state. The amount of inertia is tied to mass—the more mass an object has, the greater its resistance to changes in motion. Weight is the force of gravity on a mass, and density is mass per volume; neither of these alone describes the resistance to changing motion. So the property described here is inertia, the resistance to changes in motion.