I'm sure there is something ridiculously simple I'm missing here but this questions has been driving me crazy and now I'm sure I couldn't see the answer if it stared me right in the face. 1. The problem statement, all variables and given/known data A massless string is wrapped around a solid cylinder. A block of mass m=2.0kg hangs from the string. When released the block falls a distance of 82 cm in 2.0s. Starting with a freebody diagram calculate the mass of the cylinder. *I can't use conservation of energy to solve this as the next question indicates that I am to solve the question again using conservation of mechanical energy. 2. Relevant equations y = y0 + v0t + 1/2at2 F=ma torsion = rF Net Torsion = Moment of Inertia x rotational acceleration 3. The attempt at a solution r = ? **This is the part that has me stumped - all of the formulae I know need a radius. m = 2.0kg y = 0.82m t = 2.0s v0 = 0m/s mc = ? Step One: Determine the force of inertia of the cylinder: y = y0 + v0t + 1/2at2 a=(2y)/(t2) a = (2 x 0.82m) / (2.0s2) a = 0.41 m/s2 Fa = ma = 0.41m/s2 x 2.0kg = 0.82N Fg = mg = 9.81m/s2 x 2.0kg = 19.62N FI = Fg - Fa = 19.62N - 0.82N = 18.8N This is as far as I can get before I start running into brick walls. I've played with several different formulae but I always run into a point where I need the radius of the cylinder (which I don't have). Am I missing something really obvious here?