Bucskin, if momentum is not related to penetration, then why does an arrow continue in a certain direction when it contacts an animal? Why doesn't it take a random zigzaging course through the animal? Why does the arrow penetrate at all? SOMETHING allows the arrow to penetrate in a fairly straight line through the animal instead of the arrow simply expending all it's energy on the animal's hide and falling to the ground. The only something I know of that would do that is momentum.

Momentum is a form of inertia that allows an object, for our purposes an arrow, to resist changing it's current direction and speed.