What is inertia primarily related to?

Inertia is fundamentally the property of an object that quantifies its resistance to changes in its state of motion. It is primarily related to the mass of an object. The greater the mass of an object, the greater its inertia, meaning it will require more force to change its velocity, whether that be to accelerate it or to decelerate it. This principle is rooted in Newton's First Law of Motion, which states that an object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity unless acted upon by a net external force.

Mass is a measure of the amount of matter in an object, which directly impacts its inertial properties. Heavier objects, or those with more mass, have a higher inertia and are more difficult to start moving (accelerate) from rest or to stop once they are in motion. This relationship between mass and inertia illustrates why mass is considered the defining characteristic of inertia.