The concept of relativistic mass brings a consistency and simplicity to the 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.

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