We can only measure the initial K.E. of an arrow when shot from a bow by using a chronograph to get speed. We can't figure how much K.E. in the form of vibration is in the arrow while in flight, which I beleive is what Sylvan is talking about. Once in flight, energy is constantly being lost to distance, drag and gravity. Vibration in the arrow is also being lost in flight but has no bearing on how hard the arrow hits the target. If speed is lost, so is the momentum of the arrow. If I drop a hammer on the floor from 3 feet, there will be less momentum than if I apply downward force to the handle with my hand on the way down, from 3 feet also.