I have a cylindrical hoop of mass M and radius R with string (presumably thin and massless) wrapped around it. I take the end of the string from the hoop and let the hoop fall. (a) What is the torque about the CM of the hoop as a function of time? (b) What is the tension of the string as a function of time? For (a), I decided to use angular momentum and then using the relation [itex]\tau = dL/dt[/itex] to find the torque. I get L = RMv(t) where v(t) = gt and so dL/dt = RMg. I know something is wrong here since the torque is NOT dependent on time. For (b)...well, I can't do (b) since I need to solve (a) first. What am I doing wrong?