I will go ahead and post up, from an engineering standpoint. I am an engineer, so I am somewhat qualified.

THe two formulas M=m*V and KE=m*V^2, or KE=m*V*V. Both formulas have V or velocity/speed in them. An arrows highest speed is the second it leaves the bow. From that point forward, it is losing speed, just like a bullet. Your highest KE and M are at that first instant. Where things get dicey is the point of impact. If that arrow is flopping like a fish, the ability to transfer that KE or M in a straight line is reduced, as a function of the amount of deflection from straight. Basically you decrease efficiency at which that arrow can continue in a straight line. There is a finite amount of energy available to a projectile. If the projectile if flying straight, more of that available energy is spent continuing the path forward. The more side to side, the more energy you lose. It is the opposite of mechanical advantage. There is no more engergy being input into the projectile, so it is all losses. The fewer the losses, the more energy or momentum is available to continue in the straight line and overcome the resistance presented by penetration.

Hope this made sense, or was at least as clear as mud. I have not proof read yet, as I was in a bit of a hurry, but I can add more later.

JMAC