1. The problem statement, all variables and given/known data A person stands on a bridge that is 100 m above a river and attaches a 30-m-long bungee cord to his harness. A bungee cord, for practical purposes, is just a long spring, and this cord has a spring constant of 40 N/m. Assume that your mass is 80 kg. After a long hesitation, you dive off the bridge. How far are you above the water when the cord reaches its maximum elongation? 2. Relevant equations Uspring, initial + KEinitial + Ugravity = Uspring, final + KEfinal + Ugravity kx = mg 3. The attempt at a solution So my main problem with this question is that if I use the law of conservation of energy, where Uspring, initial + KEinitial + Ugravity = Uspring, final + KEfinal + Ugravity, I get a different answer for this question than if I set the force of the string equal the force of gravity. For spring problems such as these, can anyone clarify for me when to set the force of the spring equal to the force of gravity, or when to use the law of conservation of energy? Thanks!