1. The problem statement, all variables and given/known data A ball of mass m travels with speed v, hits a stationary ball with the same mass m and after collision they both move at speed v/2. From the point of view of the first ball the total momentum is -mv and after the collision it is 0. why isn't the law of conservation of momentum valid for this coordinate system? 2. Relevant equations The first and second laws of newton, F=ma and that a body is at rest while no forces act on it are valid in all coordinate systems moving at constant speed. 3. The attempt at a solution The coordinate system has acceleration during the collision so F=ma isn't valid. i don't know to continue from here and i guess it's the right answer, no?