1. The problem statement, all variables and given/known data A disk (yo-yo) of Radius R and mass m is attached to a weightless cord on the disk's edge. At time zero, the disk is released and a force upwards is applied to the upper end of the chord so that the center of mass of the disk does not move up or down. What must be the minimum length, L, of the cord wound around the disk at time t=0 if you wish to keep it hovering this way until time t after you release it? 2. Relevant equations 3. The attempt at a solution I can't recall how the torque produces an upwards force on the disk. How does that work again? Thanks in advance.