1. The problem statement, all variables and given/known data 5 identical masses M are suspended by a spring stretched a distance of L. If 3 of these masses are removed, what is the potential energy stored in the spring? 2. Relevant diagram So L is the distance stretched with 5 masses. Let L2 be the distance stretched with 2 masses remaining. L2 < L since the spring would "shrink" if it had to hold up less mass. L2 = (2/5)L The potential energy is E = ½(k*x2) 3. The attempt at a solution The answer which the book gives is (5/2)MgL Which would make sense if we are measuring the potential energy when the spring is holding 5 masses. Since F = 5Mg = kx, and x = L, so E = ½(k*x2) = ½(5MgL) = (5/2)MgL But isn't the question asking the potential energy when the spring is holding 2 masses? As I understand it, the potential energy would change: F = 2Mg = kx, and x = (2/5)L, so E = ½(k*x2) = ½(4/5MgL)=(2/5)MgL Which answer should it be?