1. The problem statement, all variables and given/known data. An atom at rest can undergo radioactive decay, ejecting an electron at a maximum speed of 0.5c. If the atom in a particle accelerator is observed to produce an electron traveling at 0.75c, at least how fast must the atom itself have been moving? 2. Relevant equations u0 x =(ux −v )/(1−uxv/c^2) 3. The attempt at a solution The problem is asking for the speed of the atom, I set ux=0.5c v=0.75 After plugin in, I get the result of -0.4c I don't feel sure aboout this anwser. Any suggeestions? Corrections?