In the September 2020 issue of TPT, Edw. S. Ginsberg presents a simple derivation of Einstein’s addition of velocities formula starting from the expression for the radial Doppler shift. However, this result can be found in the literature as early as 1962 as part of Hermann Bondi’s “common sense” approach to special relativity.1 His method is referred to as the “k-calculus”2 (k being the Doppler shift) and its merits are discussed in Refs. 3 and 4. Although the k-calculus has obvious pedagogical value, it does not seem to be widely known. I hope this letter inspires more teachers of special relativity to consider using it in their classrooms.

AAPT members receive access to The Physics Teacher and the American Journal of Physics as a member benefit. To learn more about this member benefit and becoming an AAPT member, visit the Joining AAPT page.