After reading all 4 pages of this post, I still haven't seen the formula that was taught to me in my college physics class.

The formula for kinetic energy is 1/2 mass X velocity squared.

Mass and weight are technically different but it is considered the same in this formula. (it is different in space with zero gravity)

Velocity is your feet per second FPS.

Back a ways in this post, someone wrote that speed is more important. They are correct, because you multiply the speed by itself which increases its value A LOT.

On the other hand, the weight is less important because before you multiply it with the speed, you cut it in half.

Let's consider an arrow that weighs 500 grains and flys at a speed of 200 FPS. You multiply 200 times 200 and get 40,000. You then multiply this times half the weight which is 250, and you get a VALUE of 10,000,000.

Now let's consider an arrow that weighs 300 grains, BUT flys 320 FPS. 320X320=102,400
Then multiply this 102,400 times half the weight (150) you get 15,360,000!

This is an increase of more than 50%

That is why speed is more important than weight when it comes to KINETIC energy.

I am in NO way saying that the only way to go is to use lighter arrows, I am only giving an example of how to find kinetic energy in a bow. I am also not saying that a heavy slower arrow won't kill deer, this is only here to teach how to find the kinetic energy.