Derivation of Relativistic momentum

[trsomas]

Only if you can prove that the momentum of a system is conserved in one inertial frame, you can claim that it will be conserved in another inertial frame too. In this video, the momentum has not been shown to be conserved in any inertial frame.

[trsomas]

There is something wrong with the derivation. You have shown that the relativistic mass depends on the relative speed between the frames. But, in reality, it depends on the speed of the object whose relativistic mass we have to find.