I would like to understand velocity addition and subtraction in special relativity, more than I currently do. It would be greatly appreciated if one could comment on the outcome of the following thought experiment. Imagine ‘the super-tank’ a tank capable of speeds of 0.45 C is being designed. Someone on the design team, raises a potential problem. When considering the tanks tracks, the tracks that are in-contact with the ground or the bottom-side tracks, have no velocity until the tank moves over them and pulls them to the top-side. The velocity they have according to a stationary observer is twice the speed of the tank. The member of the design team states that if you treat the tank as the stationary observer, then what ever speed the top-side has the bottom-side needs to have, just in the opposite direction. If this is not the case then the tank tracks would rip apart, as either the top-side or bottom-side tracks would not feed enough track to the other. It is then shown that the tank will not have the same but opposing velocity for its tracks, in the reference frame of the tank, based on the velocity of the bottom-side tank relative to the ground observer being 0 and the top-side 0.9C. U = (S-V) / (1-(SV/C^2)) U: The velocity of the tracks according to the tanks frame of reference. S: The velocity of the tracks according to the ground frame of reference. V: The velocity of the tank according to the ground frame of reference. C: the speed of light in a vacuum. Bottom-side velocity according to the tank U = (0 - 0.45C) / (1-(0C*0.45C/C^2)) = -0.45C / 1 = -0.45C Top-side velocity according to the tank U = (0.9 - 0.45C) / (1-(0.9*0.45/C^2)) = 0.45C / (1-0.405) = 0.45C / 0.595 = 0.7563C How can this difference exist considering the concerns of the designer?