What does inertia refer to in physics?

Inertia is defined as a property of matter that quantifies a body's resistance to any change in its state of motion. This means that an object at rest will remain at rest, and an object in motion will continue moving at a constant velocity unless acted upon by an external force. This principle stems from Newton's first law of motion, commonly referred to as the law of inertia.

Understanding inertia is crucial in physics because it elucidates why objects behave the way they do when forces are applied. For example, when you push a stationary car, the car’s inertia resists the initial force you apply, which often requires a sufficient amount of effort to overcome. Similarly, when a vehicle suddenly stops, the passengers lurch forward due to their inertia, seeking to maintain their state of motion.

The other options do not adequately describe inertia. While mass in motion refers to momentum, it does not capture the concept of inertia itself. The force of gravity pertains to attraction between masses and is unrelated to the definition of inertia. Energy in motion could imply kinetic energy but does not reflect the resistance aspect that defines inertia.