When reading lessons on the conservation of momentum, you usually see examples with colliding balls or something to that effect. These examples always seem to fail to mention friction. These balls will always come to a stop due to friction. How is momentum conserved when it is lost to friction? In a system of with an initial net momentum, where does the momentum go once the momentum of all the macro-cosmic objects is reduced, by friction, to 0? I know momentum is conserved for closed systems. Many people might simply say that the friction is an external force. However, you should be able to choose your system so that it includes the source of friction. Part of me wants to say that the momentum dissipates into the microscopic movement of the atoms in the surface which caused the friction. However, I've also heard that the momentum vector of atoms cancel out as a result of friction. Any help will be greatly appreciated it. I've been racking my brain about this one for the past few days.