Derivation of velocity dispersion from virial theorem?? Hey, Im probably being a bit dim here but could anyone help me derive the velocity dispersion from the virial theorem. Ive got 2K+U=0, K/m~sigma^2 and U/m~GM/R. From rearranging I get a negative velocity^2? Or maybe its the gravitational force thats negative. Oh and a factor of 2. Is that just ignored?? My notes say the answer is sigma^2~GM/R Just a bit confused, anyone got any clues?