1. The problem statement, all variables and given/known data The stretch of a nonlinear spring by an amount x requires a force F given by: F=40x-6x^2 where F is in Newtons and x is in meters. What is the change in potential energy U when the spring is stretched 2m from its equilibrium position? 2. Relevant equations U=.5kx^2 = .5(kx)x F=kx 3. The attempt at a solution F=40(2)-6(2)^2=80-24=56N U=.5(56N)(2)=56J Thats what I thought would work, but 56J is no the correct answer. What am I doing wrong?