Hi, I have yet another question in the field of the special relativity theory. I always thought that the derivation of the relativistic mass is trivial. But I discovered that there is some complexity due to the transverse and longitudinal relativistic mass derivation. I saw a thread where people talked about different aproaches which can demonstrate that the transverse relation must be used for relativistic mass calculation. For me it always seemed normal to only consider the transverse relation (in y direction). Since we have the length contraction phenomena in x direction which "compensate" the difference between transverse and longitudinal 2 way light travel. I'm not an expert and maybe my vision is too simplistic but i would like to know if this arguments are sufficient for demonstration of transverse choice.