Science of Archery: Physics of the Bow and Arrow
Archery involves physics far more than is immediately evident to the eye. The amount of kinetic energy that is stored in both the bow and the arrow itself is a fairly complex calculation. While the actual numbers are useful in determining the amount of force and speed one can expect considering the draw weight of the bow, the spine weight of the arrow, and the transference ratio of the stored energy by the string and cams to the arrow, what this all boils down to is accuracy and penetration for the average archer.
When the bow is draw back to full draw, the act of pulling back the string transfers the energy to the limbs of the bow. The cams, or wheels through which the cables are threaded, significantly reduce the amount of strain that it takes to keep the string held back in a full draw position. The energy is still stored within the bow limbs, but the effective draw strength is reduced dependent upon the cams utilized on the bow,
When the arrow is released, that stored energy of the stressed limbs is transferred through the cams and string; effectively propelling the arrow forward at great speed. The weight and stiffness of the arrow, referred to as the spine of the arrow, then reacts to the kinetic energy loaded upon the nock end of the arrow with a dynamic linear flexing process that actually causes the arrow to bend and oscillate in flight. This flexing starts almost immediately as the shaft of the arrow slides along the arrow rest. Fine tuning the rest to the rate of oscillation will keep the arrow flying predictably and hence more accurately.
The fletching or vanes, help stabilize the flight of the arrow by imparting a spin or twist to the shaft. Acting as small rudders, they eventually straighten and steer the arrow towards the target overcoming the oscillation of the shaft itself. This rotation of the shaft in mid-flight helps to counteract the effects of wind and air pressure upon the arrow and provide a more stable and predictable path.
Upon the arrow hitting the target, relative penetration then becomes a calculation of mass weight and arrow speed with higher values in either category providing deeper penetration ratios. Where a complete pass through shot is preferred to increase the damage canal and allow for quicker blood loss, the higher the penetration ratio, the more lethal then is the shot.