A rectangular block slides down the face of a wedge. A string runs from the block, over a wheel at the top of the wedge, and to a wall, so that as the block slides down it displaces the wedge toward the wall. This Demonstration shows how the system acts once the block is released. The acceleration of the block with respect to the wedge must be the same as that of the wedge with respect to the ground.
Since the acceleration is a constant in time, it must be directly proportional to the displacement; this is shown by a colored segment on the ground. The maximum length of the string is limited so that the corner of the block can just touch the ground.