There is a line written in my physics text book it says " For any type of collision momentum can be conserved before and after the collision about the line of collision and and about the line perpendicular to the line of collision" . Well i got a question here can we always conserve momentum about the line perpendicular to the line of collision? Since all this time i knew we could only conserve momentum about the line where no net force is acting. If two projectiles collide head on in mid air then along the vertical direction i.e along the line perpendicular to the line of collision "mg" is acting Hence net force is not ZERO. So how ?