If bow needs to be very powerful, keeping the final string angle small by making the bow long is a good idea. The difference is illustrated with the two (made-up) F/D curves shown below the shorter bow stores 8019 units of energy, whereas the longer bow stores 59% more energy (12741 units): Similarly, a short bow will have a very concave F/D curve and thus it stores much less energy. This means that a long bow (such as the English longbow) stores a lot of energy. Very long straight bows will have a relatively flat F/D curve: this is because the string angle does not change as much as with shorter bows. The concavity of the curve depends on the ratio between bow's length and it's draw length. This is because more energy is required to draw the bowstring back the same amount at the end of the draw than at the beginning. In real bows, however, the concavity of the curve is apparent. The bow in question would store 14000 units of energy: A theoretical best-case bow's F/D curve is a straight line and is illustrated below. In simple straight bows with no special features such as recurved tips the force-draw curve is more or less concave. For bow design purposes it's the shape of the curve that's most important the actual units or numbers are not as relevant. The area that's left below the curve approximates the energy that's stored in the bow. A force draw curve is plotted by measuring the draw weight of the bow at several points along the entire draw length. The amount of energy stored in a bow can be calculated by plotting it's force-draw or F/D curve. When the bowstring is released, this stored (potential) energy is converted into kinetic energy of the projectile (among other things). The farther it's drawn, the more energy is stored.
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