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than the load moves.
    Wheel and axle : A longer twist at the outer edge of the wheel exerts a more powerful but shorter motion near the axis.
    Screw : Applying a larger but easier rotary force creates a smaller forward motion.
    PHYSICS OF THE BOW AND ARROW
    The bow and arrow was used extensively in early warfare. Archers were trained from an early age. In some cases they advanced toward the enemy on foot, carrying a shield; in others cases they rode in chariots. As we saw in the last chapter , chariots usually had a driver and an archer, and when the chariot got close enough to the enemy, the archer would begin firing arrows as fast as he could.
    A bow, in essence, is a simple machine that changes one type of energy into another, making it easier for the archer to give the arrow a high velocity. What is needed for a high velocity is a rapid and forceful arrow movement, and of course muscles can do both of these, but not at the same time. To understand the physics of the bow and arrow let's begin with the archer loading an arrow and pulling the string back slowly. He is using his arm muscles to do this. He pulls the string back to its maximum extension, and in the process the bow bends. The energy from the archer's muscle contraction is stored in the bending of the bow. This is potential energy. He then lets go of the bowstring, at which point the string moves rapidly to the normal rest position. In the process it transfers energy from the bow to the arrow. In essence, potential energy is transferred to kinetic energy, as in the case of a falling ball. The transfer is obviously very rapid, and this gives the arrow a high speed. Note that the archer has produced a certain amount of energy, and by the conservation of energy, this energy must remain constant. But the bow can move with both a high force and a high velocity in a way that his arm cannot. The bow is a machine that stores energy. Muscle power is used to load the machine at low speed, then the machine releases the energy at high speed. Indeed, if you know all the variables, such as the mass of the arrow, the distance the bow is pulled back, and what force it exerts, you can equate potential energy to kinetic energy and determine how fast the arrow will leave the bow. Furthermore, if you know the angle at which is it is aimed (and ignore air pressure) you can determine how far it will go. 7
    Over the years bow and arrows were gradually improved. Several factors are important in relation to how powerful a bow will be. Three of the most important are its length, its shape, and its composition. In general, the longer the bow, the more powerful it will be, but other factors play a large role. We will see later that the English developed a very effective longbow and used it with considerable success against the French.
    The overall shape of the bow is also important. Early bows had a single curve and were made of wood. Eventually, however, archers determined that if the ends of the bow were curved away from the user, the arrow would go farther. This was because the curving shortened the distance between the bow and the string at rest, and as a result, the string traveled farther before coming to a stop as it released the arrow. This extra push gave the arrow a little more momentum and speed. This type of bow is called a recurved bow.
    The bow's composition was, of course, also critical. The type of wood, or other material, it was made of had a large effect on its power. Also, a bow's density, elasticity, and tensile strength (amount of stress it can take before it breaks) determines how much energy it can store and how well it returns to its original shape after the shot.
    Early on it was discovered that bows made from more than one material were more effective than simple wood bows. They are referred to as composite bows. Composite bows were usually made of wood, a section from the horn of an animal, and sinew. A thin section of horn was glued to the belly of the bow on the