Inertia is defined as...

• A. lack of activity
• B. a property of moving quickly
• C. an emotional state
• D. a type of material

Answer: (A)

Inertia is the resistance of any object to a change in its motion or rest. An object at rest tends to stay at rest, and an object in motion tends to continue moving in a straight line at the same speed unless a net external force acts on it. The option that best embodies this idea in everyday language is the notion of a lack of activity, since inertia describes not changing the current state of motion. The other choices miss the concept: inertia isn’t about moving quickly, it isn’t an emotional state, and it isn’t a type of material.