I don't understand why momentum is consesrved in each of the 2 cases below: Case 1) A 10g ball and a 30g ball are placed in a tube with a massless compressed spring between them. When the spring is released, the 10g ball flies out of the tube at a speed of 6.0m/s. With what speed does the 30g ball emerge from the other end? Case 2) Bob sees a staionary cart 8.0m in front of him. He then accelerates steady at 1.0m/s^2, jumps on the cart, and roll down the street. Bob has a mass of 75kg and the cart's mass is 25kg. What is the cart's speed just after Bob jumps on? I know perfectly how to solve these 2 problems, just apply the momentum conservation law, the mechanical calculations are easy, but for momentum to be conserved, the system must have no net external forces. Now, it's like I'm blindly applying momentum conservation law without understanding it because I'm not sure whether momentum is conserved in each case, and if so, why it's conserved. Can somebody kindly explain WHY and HOW momentum is conserved in each case and in which "system" is the momentum conserved in each case? (please don't solve the problems because I know how to do so...) Any help is greatly appreciated!Please Register or Log in to view the hidden image!