Hi, I just want to understand this concept a bit better. The law states that momentum is conserved when there is no external force acting on the system. Now consider this situation where two cars of equal mass and moving at equal speed collide head-on to come to rest. Would the momentum be conserved? Looking at the situation, there is friction acting on the cars so does that mean momentum is not conserved? Similarly for this situation: a bicycle rider ceases to pedal and her bicycle coasts along the path until it comes to rest. Is momentum not conserved as well due to friction? Many thanks.