1. The problem statement, all variables and given/known data This is a conceptual/logic question I have. I don't understand why for minimum P: P must be perpendicular to R; angle beta = psi_s. (please see attached picture). Once I can get my arms around that statement, I can solve the problem. Thanks for any logic help. Or if there's another way to approach this, that's cool too. 2. Relevant equations 3. The attempt at a solution Why is the angle Beta not = 0? That is the most puzzling thing conceptually. It would seem that if angle Beta=0, you would get the most mechanical efficiency because the direction of force P would then be perpendicular to the normal N.