A block of mass 0.4 kg on a horizontal surface is attached to a horizontal spring of negligible mass. The other end of the spring is attached to a wall, and there is negligible friction between the block and the horizontal surface. The center of mass of the block is stretched a horizontal distance x=0.25 beyond its equilibrium position and released from rest such that the block-spring system undergoes simple harmonic motion. A graph of the spring potential energy of the block-spring system as a function of the horizontal position of the center of mass of the block is shown. The positive direction is considered to be to the right.