in here: http://www.physicsclassroom.com/class/newtlaws/Lesson-1/Inertia-and-Mass Mass as a Measure of the Amount of Inertia All objects resist changes in their state of motion. All objects have this tendency - they have inertia. But do some objects have more of a tendency to resist changes than others? Absolutely yes! The tendency of an object to resist changes in its state of motion varies with mass. Mass is that quantity that is solely dependent upon the inertia of an object. The more inertia that an object has, the more mass that it has. they say mass is a measure of inertia... why is that the case? why isn't inertia a measure of mass?