Common examples of the conservation of momentum center around two or more objects colliding in the absence of external forces. Another twist on the example of a collision would be the idea of a reverse collision or an explosion. If you video tape an explosion and play it backwards, it looks like a collision. Likewise, if you video tape a collision and play it backwards it resembles an explosion.
If we look at the case of a canon being fired, we find there is a force of the gun powder exploding creating a force of the canon pushing on the cannon ball and the cannon ball pushing back on the gun. These two forces are equal in size but opposite in direction. If any forces that are external to the cannon-&-ball system (such as weight and friction) are removed. Then momentum must be conserved.www.regentsprep.org/Regents/physics/phys01/expmomen/default.htm