What quantity measures the resistance to a change in motion?

Inertia is defined as the property of an object that resists changes to its state of motion. It is an inherent characteristic of matter that depends on the mass of the object. Essentially, inertia means that an object at rest tends to stay at rest, and an object in motion tends to stay in motion at a constant velocity unless acted upon by an external force.

While net force and friction play roles in motion and resistance, they do not directly quantify the object's resistance to changing its motion. Net force refers to the total force acting on an object, determining its acceleration according to Newton's second law, and friction is a force that opposes motion but does not represent the inherent property of inertia itself. Mass is related to inertia, as greater mass indicates greater inertia, but it is not the direct measure of resistance to motion. Thus, inertia is the specific term that encapsulates the idea of resistance to changes in motion.