1. The problem statement, all variables and given/known data 2. Relevant equations v = w x r 3. The attempt at a solution I got the answer correct after a few tries, but I am little confused on why it has to solved this specific way. I know that v = w x r, so why can I not just say w = v/r? Is it because w is measured from the center of the roller so v = w x r would be vcenter, not vtop? That is how I ended up solving it, but I wanted to make sure I didn't get there "accidentally" or misinterpreted the process. Using that diagram and concept of similar triangles: tan θtop = tan θcenter Stop/2r = Scenter/r Stop = 2*Scenter, Scenter = rθ Stop = 2rθ d(Stop)/dt = vtop = d(2rθ)/dt = 2rθ' = 2rωtop thus, ω = v/2r Any reasoning as to why I have to do this would be much appreciated!!