Consider a spring that does not obey Hooke's law very faithfully. One end of the spring is fixed. To keep the spring stretched or compressed an amount x, a force along the x-axis with xcomponent Fx =kx - bx2 + cx3 must be applied to the free end, where k = 100 N/m, b = 700 N/m2 , and c = 12000 N/m3 . Note that x > 0 when the spring is stretched and x < 0 when it is compressed. (Many real springs behave in a similar fashion.)