1. The problem statement, all variables and given/known data U = Ax2 - Bx3 2. Relevant equations du/dx = 2Ax - 3Bx2 3. The attempt at a solution If I was given a potential energy function U = Ax2 - Bx3 and am asked to find: 1) The expression for the force as a function of x. 2) The equilibrium points and determine if are they stable or unstable? So, for 1): Would I differential the function giving like so? U' = f'(x) = 2Ax - 3Bx2 Now for 2): Would I set f'(x) = 0 to find the equilibrium points? f'(x) = 2Ax - 3Bx2 = 0 In return I get the points of x through the quadratic equation: x = 0 and X = A/B If this is all correct how can I determine if a equilibrium point is stable or unstable?