Regarding momentum, and the "Law of conservation of linear momentum", my book states that it's more general than mechanical energy, since mechanical energy is only conserved for conservative forces, while linear momentum is conserved independent of the forces, as long as the sum of external forces is zero. This really confuses me. Let's look at two situations: Situation 1) Collision. When a collision between two balls occur, a vibration in the air is produced (the sound wave) which wastes a part of the energy, and thus mechanical energy isn't conserved. On the other hand, there are no external forces, so momentum is conserved. Is this reasoning correct? Situation 2) A ball is rolling on a table. There is friction. Ok, so, let's pick up our system as ball + table, in this case our momentum would be (Ball Momentum = x and Table momentum = 0), the friction force would be internal to the system, and yet we would lose momentum. Now, for the mechanical energy part, friction would produce a certain heat, which would waste energy, and thus mechanical energy would also be reduced. Is my second reasoning correct? And also, why isn't momentum conserved?