If we imagine a device like one shown on the picture above, why can't we say that it would work? Here is a cylinder somehow placed in the fluid by one of his sides (one in the box). Due to additional buoyant force on the right, we should have it rotating counterclockwise, but this doesn't happen. Why? Could you explain.
Presumably the cylinder is fixed to rotate about its center; otherwise the pressure would push it up and out of the fluid.
There is no torque on the cylinder. The pressure applies a force parallel to the radius of the cylinder, so it doesn't turn.
As noted above, there IS a net force on the cylinder pointing up and away from the fluid, but the force doesn't cause it to rotate.
