Inertia is determined by an object's mass. Here's why:

* Inertia is the tendency of an object to resist changes in its motion. A heavier object (with more mass) is harder to get moving and harder to stop once it's moving.

* Speed is how fast an object is moving. While speed is related to motion, it doesn't affect the object's resistance to changes in that motion (inertia).

Think of it this way: A bowling ball and a feather both experience the same force due to gravity. However, the bowling ball (with much more mass) has significantly more inertia and will be much harder to move or stop than the feather.