A box is resting on a frictionless surface and is connected to a horizontal spring ( which is connected to a wall). The spring force that the box experiences as a function of position is shown below. For three situations, the initial and final positions, respectively, along the x axis for the box are
(a) -10 mm, 0 mm
(b) -10 mm, + 5mm
In each situation, is the work done by the spring force positive, negative or zero ?