Imagine a situation in which a man is standing on a floor. The floor is free from any external force (you can imagine it like the floor is resting on another friction-less floor), though gravity acts on the man and there can be friction between the man and the floor. If the man starts running in one direction, the floor starts moving in the opposite direction to conserve the linear momentum.
But to a fixed observer outside the "floor and man" system, the entire system moves as the floor (with the man on it) moves in the direction opposite to the direction of running man. If the observer cannot see the running man (suppose if everything above the floor is curtained) then it appears as if the system gained momentum without any external force acting on it. Isn't this a violation of the law of conservation of linear momentum? Can anybody explain the situation, please?