1. The problem statement, all variables and given/known data A disk of mass M and radius R is help up by a massless string. Let there now be friction between the disk and the string, with coefficient u. What is the smallest possible tension in the string at its lowest point? The problem is problem 8 from here: http://www.personal.kent.edu/~fwilliam/Chapter 1 Statics.pdf 2. Relevant equations Fnet = 0 at equilibrium. fs <= uN 3. The attempt at a solution I understand how to do the problem when the angle theta is defined to be 0 at the lowest point and pi/2 at the right end. In this case, the result is T(0) >= Mg/2 e^(-u*pi/2). However, I wanted to see what would happen when theta is 0 at the right end and -pi/2 at the lowest point since it shouldn't matter. As in the solutions, the equation for tension as a function of theta should still be T(theta) <= T(0) e^(u*theta). In this case, T(0) = Mg/2 so T(theta) <= Mg/2 e^(u*theta). So if you plug in theta = -pi/2, T(-pi/2) <= Mg/2 e^(-u*pi/2). But here, the direction of the inequality is different. Was I supposed to switch the sign somewhere?