New member here... I have the following homework problem: "A 200-g mass attached to the end of a spring causes it to stretch 5.0 cm. If another 200-gmass is added to the spring, the potential energy of the spring will be" I know the answer is 4 times as much but I am not quite sure how the answer is being reached. I thought I could use the forumla: U= 1/2 kx^2 But I am wrong. Any help would be much appreciated! Thank you!!! A.