Hello everyone, For some time I'm a little bit confused about (at the first view) a very simple question, which is about the conversation of the energy of moving objects (in terms of special relativity). As an example lets talk about firearms. If the mass of the gun M1 is infinitely higher than the mass of the bullet M2, then all the kinetic energy of the shot will be imparted to the bullet. Now, lets take the case of the moving gun-bullet system (in X-direction). In the frame of the static observer (S) the total velocity of the bullet (or kinetic energy) will be different as a function of the direction of the shot (transverse or longitudinal); since sqrt(Vx2 + Vy2) < Vx + Vy. As far as I understand the total kinetic energy of the system is independent of the direction of the shot, then how one can explain the total energy difference between transverse and longitudinal shot? My first guess was that the answer was hiding in the distribution of the energy between the gun and the bullet; however I estimate that this guess is wrong since M1 (gun) >>>>> M2 (bullet) ->> all the energy of the shot is imparted to the bullet.