Momentum is not conserved because energy has been added. Are you saying that a land vehicle with a horizontal fan to drive it along rails will not accelerate and move? Will the vehicle not build up momentum because of this?

Momentum is always conserved. If you see momentum disappearing, you are
not looking at the whole system. In the case of the land vehicle
propelled by a fan, the air blown back acquires momentum in one
direction, exactly balanced by the momentum that the vehicle acquires,
plus the (rotational) momentum (due to wheel friction) that the earth
acquires.

Jose
--
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