In defense of relativistic mass, T.R. Sandin, Am. J. Phys. 59(11), November 1991
The concept of relativistic mass brings a consistency and simplicity to teaching of special relativity to introductory students. For example, E = mc2 then expresses the beautifully simplifying equivalence of mass and energy. Those who claim not to use relativistic mass actually do so--if not by name--when considering systems of particles or photons. Relativistic mass does not depend on the angle between force and velocity-- this supposed dependence results from incorrect use of Newton's second law of motion.