by Darmon Richter

Created on: May 23, 2009

The bows used in archery work by converting the potential energy stored in the limbs, into the kinetic energy of the flying arrow. There will always be some energy lost in the process, in this case through a process known as elastic hysteresis, as well as dampening effects from the string and the limbs of the bow. In some cases the arrow itself will bend or compress as it is drawn, absorbing more energy still. The effect of this is to cause an in-flight oscillation along the length of the arrow.

The design of the bows themselves has evolved much over the years, to adapt to the limitations of the mechanics behind archery. Compound bows for example, are designed to require less power to pull the string and hold it taut. Usually this is achieved using elliptical wheels or cams on the limbs, which are designed to allow an archer to draw and hold twice as much weight.

The aim in archery is also subject to potentially adverse physical laws; an arrow which doesn't have enough spine, or flexibility for the bow will be unable to flex around it, so that as the string snaps back, the arrow will be forced sideways, rather than bending past the bow. If the arrow has too much spine, it can be propelled off in the opposite direction.

This leads to what is known as the 'Archer's Paradox'. The concept, coined in the 1930s by Robert Elmer, is based on the idea that the flexing of a drawn arrow-shaft means that the archer must aim off to one side in order to hit a target. This problem can be overcome however, by a number of methods: using a centreshot bow, a drop-away arrow rest, the pressure button designed by Vic Berger, or by using arrows with the correct spine, or stiffness, for the bow you are using. The intention is that a shaft will be flexible enough that after being steered off-target by the curve of the bow, it will rebound back the other way just enough to strike the target accurately.

Once the arrow is in flight, a sure aim can further be assured by the arrow's fletching. Good fletching will ensure that a restoring torque will bring the arrow back on track if it strays too far from its vector of travel. Some arrows are fletched so that the shaft rotates in flight; this is done to eliminate any directional pressure build-ups that might influence the arrow to fly off-course.

The flexibility of an arrow's spine will also need to be adjusted in keeping with the strength of the bow. An overly powerful bow may have the effect of compressing the shaft as the string recoils, or accelerating it too sharply as it moves around the bow. Less powerful bows will have a less deformative effect on the arrow, and so may be used with arrows which have less spine.

The straightness of the shaft will also have a direct effect on speed. A straighter, narrower arrow will incur less air resistance, and so the arrow will travel faster. The shape of the tip is another factor with a strong influence on the arrow's speed, those with a sharper, pointed tip being able to travel faster than blunt ones.

The other effect of different arrow tips will occur on hitting a target; tips can be shaped so as to either concentrate or disperse force on impact. Whereas a thin, sharp arrow tip would be suitable for piercing mail or armour, a blunted one might be used in practice. A range of spiked, barbed and expanding tips have been used over the years for hunting wild animals; in this situation, the aim is to create deep, piercing wounds, allowing for maximum blood loss.

It is through developing an understanding of the above-mentioned physical aspects of archery, that the equipment of archers has been able to evolve over the years, and adapt to the physical necessities of the sport.

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