1. The problem statement, all variables and given/known data A 0.4 kg object rotates in a vertical circle at the end of a 0.5 m string. What is the tension of the string at the bottom if the angular velocity there is 8.0 rad/s? 2. Relevant equations centripetal acceleration = R*w^2 weight = mg R = radius of circle w = angular velocity 3. The attempt at a solution centripetal acceleration = (0.5 m)(8.0/s)^2 = 32 m/s^2 total (centripetal) force = (0.4 kg)(32 m/s^2) = 12.8 N (upwards) weight = (0.4 kg)(9.8 m/s^2) = 3.9 N (downwards) Upwards forces and accelerations will be considered positive, downwards ones negative. total (centripetal) force = string tension - weight 12.8 N = string tension - 3.9 N string tension = 12.8 N + 3.9 N = 16.7 N In general terms: string tension = mg + mRw^2 Unfortunately, the "official" solution is 13 N. Am I wrong, or are my course materials wrong? Will I be equally puzzled if I try to take the AP Physics B exam?