This is an annoying problem for which I have a close answer to, not sure where I went wrong. 1. The problem statement, all variables and given/known data Cannonball Man (mass = 75kg) is stuff into his circus cannon, compressing a giant spring by 1.5 meters. He is launched vertically upward at 5 m/s reaching a maximum height above the circus floor of 10 meters. What is the spring constant of the spring? 2. Relevant equations Conservation of Energy "Master Equation," as I like to call it: (1/2mv^2 + mgh + 1/2kx^2)initial = (1/2mv^2 + mgh + 1/2kx^2)final Where k = the spring constant x = compression/extention of spring h = the change in height 3. The attempt at a solution I think I went wrong with the final and initial velocities, but I'm not sure specifically where my error was. First I cancelled stuff: (0 + mgh + 0)initial = (1/2mv^2 + 0 + 1/2kx^2)final Then I plugged in my variables (75)(9.8)(10) = (0.5)(75)(5^2) + (0.5)k(1.5^2) And solved for k: 5700 N/m All I know for sure is that I'm relatively close to what my answer should be; I recieved 17/20 for my entire submission (this was an old written assignment; I have no way of checking for the actual complete answer). Can someone help me out please?