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A bow is a weapon that projects arrows powered by the elasticity of the bow. Essentially, it is a form of spring. As the bow is drawn, energy is stored in the limbs of the bow and transformed into rapid motion when the string is released, with the string transferring this force to the arrow. Bows are used for hunting, sport (target shooting), or as a weapon of war.
The technique of using a bow is called archery. Someone who makes bows is known as a bowyer, and one who makes arrows a fletcher. Together with the atlatl and the sling , the bow was one of the first ranged weapons or hunting tools which used mechanical principles, instead of relying solely on the strength of its user.
The development of gunpowder, muskets, and the growing size of armies (and their consequent demand for less-trained levies) slowly led to the replacement of bows as weapons of war, supplanted by firearms, which were simpler for conscripts to learn and use, causing bows to be relegated to sport and hobby use.
The basic elements of a bow are a pair of curved elastic limbs, typically of wood, connected by a string. By pulling the string backwards the archer exerts compressive force on the inner section, or belly, of the limbs as well as placing the outer section, or back, under tension. While the string is held, this stores the energy later released in putting the arrow to flight. The force required to hold the string stationary when pulled is often used to express the power of a bow, and is known as its draw weight. A higher draw weight is associated with a more powerful bow, which is able to project arrows heavier, faster, or a greater distance.
In bows drawn and held by hand, the maximum draw weight is determined by the strength of the archer. The maximum distance the string could be displaced and thus the longest arrow that could be loosed from it, a bow’s draw length, is determined by the size of the archer. For bows drawn and held mechanically, the maximum draw weight was a matter of engineering. The mechanical force required to draw the string was mainly limited by the time required to do so.
The elastic qualities of the limbs of a bow provide a wide variety of performance characteristics. Longer limbs typically provide longer draw lengths, but the archer may have other constraints that limit the possible size of the bow; a mounted archer usually prefers something smaller than the head-high weapon typically employed by his infantry counterpart.
The limbs of the bow must endure repeated bending into a deep curve. The tips of the limbs may consist of a single curve for the longbow, or be bent back upon themselves in the recurve bow. The recurve allows for greater draw weight with shorter limbs, but this places greater strain on the limb materials and may increase string noise when shot. Decurve bows, where the tips bend towards the archer, provide proportionally lower draw weights but may be made with relatively poor materials.
The considerable strain placed upon the limbs of the bow when drawn requires durable materials with high tensile strength and extensibility on the back of the bow, and high compressive strength and compressibility on the belly. Wood is readily obtained, reasonably good for both back and belly, and easily shaped; self bows consisting of a single material are usually of wooden construction. Considerable compromise must be made in selecting a single material, and sophisticated techniques were employed to overcome this problem. The flatbow’s cross section is shaped to spread stress more evenly, and the yew for use in English longbows was oriented to take advantage of the ability of the heartwood to store energy in compression, and the outer sapwood’s strength in tension.
A composite bow uses a combination of materials to create the limbs, allowing the use of materials specialized for the different functions of a bow limb. The classic composite bow uses wood for lightness and dimensional stability in the core, horn to store energy in compression, and sinew for its ability to store energy in tension. While composite limbs allow greater draw length with shorter limbs, they are typically less robust than a wooden limb. Composite bows are made with water-soluble glue and could be damaged by exposure to moisture.
Modern construction materials for bows include laminated wood, fibreglass, metals, and carbon fibre components.
The string that connects the tips of the limbs is under considerable stress when drawn. The string should break only at four to five times the draw weight of its bow. An ideal string material is strong for its mass, resists stretching, and remains strong after exposure to moisture.
Historically bow strings were frequently made from animal by-products such as sinew, intestine, or hair. Plant fibres such as linen or hemp were also common and usually had a superior ratio of strength to mass. When treated with beeswax they are very resistant to moisture.
Modern synthetic polymers are totally resistant to moisture, with excellent mechanical properties and very cheap to produce. Modern compound bows have a mechanical system over which the string (in the form of a steel cable) passes as it is pulled.