1. The problem statement, all variables and given/known data Ted and Mary are playing a game of catch in frame S', which is moving at 0.600c with respect to frame S, while Jim, at rest in frame S, watches the action. Ted throws the ball to Mary at 0.800c (according to Ted) and their separation (measured in S') is 1.8*1012m. According to Mary,how fast is the ball moving? 3. The attempt at a solution I am confused by this question. Ted and Mary are moving at the same speed (0.6c relative to rest frame), so whatever observed by Ted should be consistent with Mary right? Since Ted observes that the ball is moving at 0.8c so Mary should observes that the ball is moving to her direction at -0.8c.