1. The problem statement, all variables and given/known data Mr. Krane describes this scenario in a presentation of relativistic mass in his book Modern Physics John Wiley & Sons, Inc. 1983 p. 36-37 According to one observer, two balls of equal mass, m1 and m2, are approaching each other at equal speed, v. The proper mass of each is m0. They collide inelastically for a final speed of V = 0 and mass M = m1 + m2. 2. Relevant equations m' = m0 / sqrt(1 - u^2/c^2) 3. The attempt at a solution The conclusion is that the final the final mass of the two balls (stuck together) is: M = m1 + m2 = m0/sqrt(1-v^2/c^2) + m0/sqrt(1-(-v)^2/c^2) = 2m0 / sqrt(1-v^2/c^2) Instead, I argue that since two balls are now at rest in the reference frame of the observer, the mass of the two stuck balls is M = 2m0 What am I missing here?