Is it possible to have a zero vector? The question arises in the context of conservation of momentum. In center of mass reference frame, the momentum of two masses involved in an elastic collision is zero before (as well as after) collision. It is because the two masses have equal and oppposite values of momenta. Together they have zero momentum. What is the direction of this zero momentum? Since there are two possible directions opposite to one another which one do we choose. If we choose one of those directions what valid reason can we have to reject the opposite direction?