1. The problem statement, all variables and given/known data A 8.00 kg stone lies at rest on a spring. The spring is compressed 10.0 cm (.1 m) by the stone. What is the spring constant? 2. Relevant equations F = -kx E = (1/2)kx^2 U = mgh 3. The attempt at a solution The solution provided uses a net force equation balancing Hooke's Law with mg: -kx = mg k = mg/x = 784 N/m When I first tried this problem, I approached it using conservation of energy: U0 + E0 = Uf + Ef Ef = U0 + E0 - Uf = U0 + 0 - 0 = U0 = 7.84 J Where t=0 is the rock at the top of the uncompressed spring and t=f is the rock in equillibrium on the compressed spring. k = 2Ef/x^2 = 1568 N/m Using my approach, I am off by a factor of 2. Where is my misunderstanding here? I understand the solution involving net force, but shouldn't the conservation of energy work out too?