2.13. A model for a nonlinear softening spring is f(u)=ktan1usu, where k/us represents the small displacement spring constant and us is the saturation displacement. 88 MODELING MECHANICAL AND ELECTRICAL SYSTEMS (a) Compute the energy function U and the potential energy function V for this spring. (b) Plot the force versus displacement over the range 10 to 10 for the values k=100N/mm and us=3mm. Compute the work required to stretch the spring from 0 to 5mm and illustrate this graphically on a plot of force versus displacement