* Momentum is the product of mass and velocity. A fastball has a significant velocity (speed), and while a baseball isn't very massive, the combination of these factors results in a considerable amount of momentum.

* The faster the ball, the more momentum it has. A fastball thrown at 95 mph will have much more momentum than one thrown at 70 mph.

* Momentum is a measure of an object's tendency to continue moving. The high momentum of a fastball is why it's so difficult to hit and why it can cause significant force upon impact.

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