Friction as B times angular speed??! Hi, My control systems book uses a weird notation for the moment caused by friction in a rotating motor. The equation is Tfriction = B×dθ/dt Where does this equation come from? The author assumes the reader is already familiar with this equation. The only definition of friction I am familiar with is F = μN. Obviously the torque is given by T = Fd. I have never learned about torques in rotational systems so I assume the equation used in the book is well known and analogous to the translation friction equation. What exactly is the B coefficient called? Where does this equation come from? Thanks.