Woods has it right on the money. In physics, kinetic energy is a quantity that has no direction, so it is a sum of all of the energy in the arrow that is going in every direction including vibration, oscillation, heat from air resistance, etc. Momentum, on the other hand, is given a direction. In other words, momentum is the part that we as archery hunters care about in regards to penetration. Momentum is the force that must be stopped by tissue or bone, and the formula to find momentum is Weight x Velocity / 225218 (weight in grains, velocity in feet per second). The best way to increase momentum is to use a heavier arrow since the mass of the arrow determines the amount of speed lost to air resistance, vibration, etc. The more weight you can put on target the better penetration you will achieve. The "put on target" part of the last sentence is the reason that most of us have gone from heavy aluminum arrows to lightercarbon arrows. The bottom line...find a combination that you can "put on target" consistently and you'll zip through the heaviest game, but plan on edging out Mr. Murphy and his deflecting branch by choosing the extra weight.