Here's why:

* Momentum is a measure of how much motion an object has. It's a combination of an object's mass and its velocity.

* Mass is the amount of matter in an object.

* Velocity is the speed and direction of an object.

So, a heavier object moving at the same speed as a lighter object will have more momentum because it has more mass. And an object moving faster will have more momentum than an object moving slower, even if they have the same mass.