1. The problem statement, all variables and given/known data Five identical masses of mass M are suspended by a spring stretched a distance of L. If three of the masses are removed, what is the potential energy stored in the spring? 1) (4 / 25) * M * g * L 2) (2 / 5) M * g * L^2 3) (5 / 2) * M g * L 4) (4 / 25 ) * M * g L^2 5) 5 * M * g * L 2. Relevant equations Fspring = -k * x 3. The attempt at a solution Hi everyone. I get 2 to be the answer while my solutions manual says that 4 is correct. I suspect the solutions manual is wrong, but I wanted to get someone's opinion on this first. So firstly, I got the value of the "K" for the spring: Fspring = Fg of the masses k * x = 5 * M * g k = (5 * M * g) / L Then, I got the amount of stretch of the spring with only two masses. Fspring = Fg of the masses k * d = 2 * M * g [ (5 * M * g) / L ] * d = 2 * M * g solved for d = (2 /5 ) * L. Then, I took equation Uspring = (1 / 2) * k * x^2. I plugged the expressions for K and D for the stretch and I got the expression for answer 2. Have I done something wrong? Thanks in advance for the help!