The following was a question from a Work/Energy high school test. "3. An archer is shooting at a hay bale target. The archer pulls back on his bow string with 500 N of force to draw it back a distance of 80 cm. He releases the string, and the arrow (m= 1 Kg) flies straight to its target (neglect air resistance). a) How fast is the arrow traveling when it leaves the bow?" Some students used the work-energy theorem (W=delta KE in this case) and got an answer of 28.3 m/s. However, many students used the info given to determine a spring constant (k) for the bow (k = F/x), and then applied Conservation of Energy (Elastic PE = KE in this case) to the system. This yields an answer of 20 m/s for the arrow. My question is "Why doesn't the spring constant method work?"