  Relativistic addition of velocities in one dimension, though a mainstay of
introductory physics, contributes much less physical insight than it could. For
such calculations, we propose the use of velocity factors (two-way doppler
factors). Velocities can easily, often by inspection, be turned into velocity
factors, and vice versa. Velocity factors compose by ordinary multiplication.
This simple device considerably extends the kinds of questions that can be
asked and answered in an introductory course.
