box, initially at rest on a shelf at height h, is pushed off the shelf and allowed to fall to the ground. But imagine that, before pushing the box off, we have attached it to a spring anchored to the shelf that stretches vertically as the box falls. We observe that the vertical velocity of the box is zero just as it arrives at the floor. Taking our system to be the box and the spring, when the box reaches the floor the change in the system's potential energy (which is stored in the spring) is equal to