Hello Physics Forums, I have a fluid dynamics problem which appears to challenge some momentum principles. Suppose you have two submarines (like below), identical in shape, but not in mass. One of the submarines has density equal to the fluid, while the other is denser. They move at the same speed. The body of water + submarine is a closed system, and the two submarines only exist for contrast and do not interact with one another. Because one of the submarines is denser than the fluid, it's movement changes the center of mass of the system. For this reason it must exchange momentum with the fluid as it moves. The other submarine is equal density to the fluid, so its movement around the system causes no change in center of mass, and so it does not exchange net momentum with the fluid. EDIT: the center of mass does not change in either case. But, an outside observer will see a (very slight) shift in the position of the system as the denser sub moves around inside it. How come the denser submarine exchanges momentum with the system while the lighter one does not? If you divide the systems along the dotted line that goes through the propeller, there is not a difference between the right hand side of the diagram (equal velocities, equal drag, equal fluid dynamics). So it seems that the difference has to be on the left side of system (behind the propeller). Granted, the heavier submarine will need to push a larger volume of water to accelerate to the same speed as the lighter sub. But it seems like this would only change the magnitude of the momentum transfer and not eliminate it completely.