A body of weight 27 N hangs on a long spring of such stiffness that an extra force of 9N stretches the spring 0.05 m. If the body is pulled downward and released what is its period? I know how to do this question, except I am not understanding the concept entirely. First I would calculate k, the spring constant. k = F / x = 36 / 0.05 I don't understand exactly why the answer book is using only 9 N as the force. I would think that if you have 9 N alone on the spring, then the spring would not stretch but if you 9 N in addition to the 27 N already there, then can the spring stretch. So wouldn't the total force on the spring required to stretch it by 0.05 m be 36 N and not 9 N? Thank you.