A spring has a force constant of 931 N/m and an unstretched length of 8 cm. One end is attached to a post that is free to rotate in the center of a smooth table. The other end is attaced to a 1 kg disk moving in uniform circular motion on the table, which stretches the spring by 4 cm. Note: Friction is negligibe. What is the work done on the disk by the spring during one full circle?