What is inertia commonly defined as?

Inertia is best understood as the property of matter that describes the tendency of an object to maintain its current state of motion. This means that an object at rest will stay at rest, and an object in motion will continue to move at a constant velocity unless acted upon by an unbalanced force. The concept is rooted in Newton's first law of motion, which encapsulates this principle.

The definition highlights two aspects: the behavior of stationary objects and moving objects, pointing out that external forces are required to change that behavior. This understanding of inertia explains why seatbelts are essential during sudden stops in a vehicle; without a seatbelt, a person would continue to move forward due to inertia.

The other choices fail to capture the complete essence of inertia. For example, one option mentions changing the state of motion, but it does not specifically acknowledge the resistance to changes due to external forces. Another talks about forces acting only on moving objects, which doesn't accurately describe the fundamental quality of inertia. Additionally, mentioning forces opposing gravity does not pertain to inertia directly but rather addresses a separate topic in physics.