ORIGINAL: gzg38b

I've tried staying out but I can't resist.

Big Bulls, time on the string has nothing to do with kinetic energy of the arrow. What you meant to say is "length of the powerstroke" has an effect on kinetic energy. Here's how it works.

When you draw your bow, you are putting Potential Energy into the bow. At full draw, you have maximum Potential Energy and Zero Kinetic Energy. The Potential Energy is equivalent to the area under the draw force curve. For those that remember calculus, it's the Integral of the draw force curve. You are multiplying the Force X the distance at each point along the draw force curve, and adding all those numbers together. For a typical hunting bow, that would give you something like 80 ft-lbs of Potential Energy, depending on the draw weight and draw length of the bow.

When you release the arrow, you convert the Potential Energy into Kinetic Energy. Kinetic Energy implies motion (hence the Velocity term in the equation.) The only two variables in the Kinetic Energy equation are mass and velocity. Time on the string has nothing to do with it. It only LOOKS that way because the heavier arrow is slower. The reason the hevier arrow is slower boils down to Newton: Force = Mass X Acceleration. If Mass goes up, Acceleration must go down for a given force.

The other variable in all this is the bows efficiency. Efficiency means what percentage of the Potential Energy at full draw gets converted into Kinetic Energy of the arrow. It's a fact that bows are more efficient with heavier arrows than lighter arrows. With lighter arrows, more energy is lost in the form of vibration and noise.

Also, it is NOT always true that the heavier arrow will always have more KE than a lighter arrow. If you plot Kinetic Energy on the Y axis and arrow weight on the X axis, you will see that there is an optimal arrow weight that provides MAXIMUM kinetic energy. If you have On Target, try different arrow weights from 300 to 600 grains out of your bow and notice how at first Kinetic Energy increases as the arrow gets heavier. But if you get TOO heavy, KE will start to decrease. But MOMENTUM will continue to increase as the arrow gets heavier (also up to a point).

And MOMENTUM, not Kinetic Energy, is what matters when an object hits another object. Momentum is Mass X Velocity. In this equation, velocity doesn't matter nearly as much as it does in the KE equation. Mass is much more relevant when talking about momentum versus kinetic energy.

Now what does this all mean to an average bowhunter? Not a whole lot since just about any arrow that can be safely shot from a 50-70 pound bow will pass through North American animals. But if I were going to Africa, I'd definitely be using 600 grains arrows over 300 grain arrows!