Which statement describes inertia?

• A. Inertia is the force causing motion
• B. A body at rest will stay at rest and a body in motion will stay in motion unless acted upon by an external force
• C. Inertia means objects move in straight lines
• D. Inertia means velocity is constant

Answer: (B)

Inertia is the resistance of an object to changes in its motion or rest. Mass reflects this property—the greater the mass, the harder it is to change how the object is moving. This is why, according to Newton's First Law, without any net external force, an object at rest stays at rest and an object in motion keeps moving in a straight line at constant speed. External forces such as friction, gravity, or a push are what can alter that motion, changing either the speed, direction, or both. So the statement that best describes inertia is that motion persists unless an outside force acts on it. Inertia is not a force itself, and an object’s velocity won’t necessarily remain constant or straight if external forces are present.