1. The problem statement, all variables and given/known data Imagine two motorcycle gang leaders racing at relativistic speeds along perpendicular paths from the local pool hall, as shown in Figure 1.21. How fast does pack leader Beta recede over Alpha’s right shoulder as seen by Alpha? Solution Figure 1.21 shows the situation as seen by a stationary police ofﬁcer located in frame S, who observes the following: 2. Relevant equations u'_x = (u_x - v)/(1 - v*u_x/c^2) u'_y = u_y/γ*(1 - v*u_x/c^2) 3. The attempt at a solution I can follow the solution shown in the attachment. The problem is that I'm trying to verify the solution by attaching the s' frame of reference to the beta gang leader instead of the alpha gang leader. The relative speed of alpha to beta should be the same as the relative speed of beta to alpha, but that's not what im getting. Can you guys tell me what im doing wrong?