Here's the key relationship:
* Mass is a measure of inertia. The more mass an object has, the more inertia it has. This means it's harder to start it moving, harder to stop it once it's moving, and harder to change its direction.
Think of it this way:
* Inertia is the "laziness" of an object. A massive object is "lazy" and doesn't want to change its motion.
* Mass is the "weight" of that laziness. The more mass an object has, the heavier its "laziness" feels.
