Through the centuries, the crossbow was a weapon that employed the laws of physics to wreak military havoc and pierce armour. The crossbow changed the odds of battle triumphs in war during the Middle Ages. One of the prime reliable records of this weapon in use in war dates to the battle of Ma-Ling in China, but even older crossbows have been found in Chinese tombs.
Early crossbows generally were bows mounted to sticks of wood, or 'wooden tillers/stocks'. The short, heavy, arrow-like projectile called a crossbow bolt traveled along a groove through the tiller. As the crossbow evolved, various mechanisms were used to pull back the string and then hold the string in place until it was ready to be fired. Early crossbows had stirrups for holding an archer's foot (maintaining a strong position) as he pulled back the strong with both hands or a hook.
Physics improved these killing tools in several way. A traditional bow and arrow required that the archer be full of strength to draw the bow and hold it steady while aiming. However, with crossbows, a weaker person could use his leg muscles to assist drawing the string. Later, various levers, gears, pulleys, and cranks were used to amplify the user's strength when pulling back the string. In the fourteenth century, European crossbows were made of steel and employed crannequins - a toothed gear attached to a crank. An archer would then turn the crank to pull the bowstring.
The penetrating power of a crossbow and ordinary bow comes from energy stored when bending the bow. Like a spring that is pulled and held, energy is stored in the elastic potential energy of the bow. When released the potential energy is converted into the kinetic energy of movement.