The momentum of an object(when it collide with another object in a rest frame) in a inertia frame of 0.6c relative to a rest reference frame. The momentum is p=γmv. where γ= 1/√(1-β^2) m= the mass of an object at the rest frame v=?? v is the velocity of the moving frame relative to the rest? But if the direction of the moving frame is perpendicular to the direction where objects collide. if the direction of the moving frame is same as the direction where objects collide. Then both of them have the same momentum? how can? but the momentum do not have direction????? Thank you.