Energy of position that is indicated by the displacement from equilibrium (how much the spring is stretched or compressed from its rest state). For a spring obeying Hooke's law, the formula is ΔPEs = 1/2(k)(x_f^2 - x_i^2) (Where k is a measure of stiffness and x_i and x_f are the initial and final displacements from the same unstretched rest state).