1. The problem statement, all variables and given/known data The problem concerns a block(mass M) hanging from a light string attached to a fixed support, that is pulled down with a force of the block's weight that produces an extension of 'l'. 2. Relevant equations The elastic P.E is half of (M*g*l).However the loss in gravitational P.E is (M*g*l), The question is where does the other half go? 3. The attempt at a solution My guess is hysterisis as in rubber.