1. The problem statement, all variables and given/known data Two cars are driving along the road at constant speeds V1, V2. At t=0 they begin to accelerate with constant acceleration a1, a2. Under what circumstances is the center of mass frame of reference inertial? 2. Relevant equations Fext=0 ==> change in momentum=0 3. The attempt at a solution It's known that if there are no external forces acting on a system, the momentum is conserved. The same applies if there are external forces present, but they cancel each other so that the net external force is 0 (correct?). Now, from the center of mass (c.o.m) frame of reference, when the momentum is conserved, the total momentum is 0. Would that apply if despite the fact that there are external forces acting on the cars, if with their acceleration, the velocity of the c.o.m is kept the same (thus making it inertial)? Or is Fext=0 necessary in order for that to happen?