Here's why:

* Inertia: Inertia is the tendency of an object to resist changes in its motion. The more massive an object is, the more it resists changes in its velocity (speed and direction).

* Mass: Mass is a fundamental property of matter that measures its resistance to acceleration.

Think of it this way: pushing a heavy box takes more effort than pushing a light box. The heavier box has more inertia and resists changes in its motion more strongly.