1. The problem statement, all variables and given/known data We have a particle in a step potential. Consider it as a plane wave traveling left to right and hitting a step potential V at x=0. Assuming it behaves like a fluid, what is the force it exerts on the wall in terms of F', where F'[x] is the derivative of the wave function? 2. Relevant equations 3. The attempt at a solution The wall has to exert a force equal to the force exerted on it so as to not move. Since we assume fluid behavior, we know the force on the wall is equal to the change in momentum over the change in time. We are given that the wave hits the wall and bounces back, so the change in momentum for a particle is just 2*h_bar*k, where we assume it's final velocity has same magnitude as the initial velocity. I don't know where to go from here. The question asks that we express F in terms of the derivative of the wave function at x=0, but I don't see it's connection with force.