Energy Transformations 1. Kinetic. You pull back the string 2. Mechanical. When you pull the string it moves the pulley 3. Potential. You are holding the string back, but not letting go. 4. Elastic. Once you let go of the string the bow snaps back in to place launching the arrow. 5. Kinetic. When the bow is snapping back in place it moves the string which launches the arrow.
Simple Machines Pulley. Reduces the force needed to pull back the bow. Lever. Returns the string back in to place which shoots the arrow.
Forces Gravity Normal Effort (lifting up bow) Effort (Pulling back the string) Elastic
How it works Compound Bow A compound bow is similar to a traditional longbow or recurve bow, except its design and material give it significant advantages, among them increased power and accuracy. Like all bows, compound bows store energy in their limbs, which are compressed as the string is drawn back. When the string is released, the limbs snap to their original shape, transferring the potential energy into acceleration of an arrow. Features The primary feature of a compound bow is the use of one or two pulleys to compress the limbs. Not only do the pulleys magnify the force applied to the string by the archer, they also provide an advantage known as "let off." A standard longbow requires nearly as much force to hold it fully drawn as it does to draw it back. In a compound bow, once the string is about 50-80% drawn, the pulleys reduce the amount of force required to continue drawing the string. At full draw, it takes very little force to hold the string back, allowing an archer to have a much steadier arm for aiming. Additionally, stabilizers and dampeners can be fastened to the front of a compound bow to reduce movement on release of the string, further increasing accuracy. Design The cables and pulleys make the compound easier for the archer to use, but the real secret to its power is in the composite materials from which it's constructed. The central part of the bow where the arrow is nocked is called the riser. It's usually made of a sturdy aluminum alloy. The limbs are made of very strong but somewhat flexible composite materials and are bolted to the riser. This combination of strength and flexibility, combined with design, gives the compound bow much more power than a traditional bow, which is usually constructed from a single piece of wood.
Improvement Plan I would put more pulleys on so it will use less force. Also I add springs to the sides so it will snap back in to place a lot faster.