Description: Mocanu's Velocity Composition Paradox Since in general u v v u this raises the question as to which velocity is the real velocity. The paradox is resolved as follows. There are two types of Lorentz transformation: boosts which correspond to a change in velocity, and rotations. The outcome of a boost followed by another boost is not a pure boost but a boost followed by or preceded by a rotation (Thomas precession). So unlike Galilean composite transformations, in special relativity, boost composition is parameterized not by velocities alone, but by velocities and orientations, so u v and v u both describe correctly but partially the boost composition B(u)B(v).