Hello! First, please, zoom in the attached file and take a look on it. Image 1 demonstrates the situation: two identical balls collide, horizontal component of velocity of both remains unchanged and vertical component becomes opposite as a result of the collision. Table in image 3 shows all the velocities in terms of a (horizontal component) and b (vertical). Now we change the frame of reference into the one, which moves to the right on the speed of a. It is demonstrated on Image 2. New velocities are calculated via the relativistic velocity addition formula and are shown in the table on image 4. Finally I calculated total momentum before (image 5) and total momentum after (image 6). Results are not equal! What did I do wrong? http://stg704.rusfolder.com/preview/20140518/7/40730757_3_1176191.png Deep thanks!