If water is incompressable, how do submarines work?
Water has a tiny compressibility, but it’s still approximately incompressible. Hydraulic pressure does increase with depth, because of the weight of the water overhead bearing down, and it’s the pressure which makes the difference. Pressure is force per unit of area. In Standard International units, one Newton of force per square meter is one Pascal of pressure. The submarine is at some depth. The boat is an extended object. Its bottom is at a greater depth than is its top, so there’s more pressure, and more total force, pushing upward than there is pushing downward. The sub has a weight. At some depth its weight downward will equal the pressure upward, and it’ll be at neutral buoyancy. The sub’s weight can, of course, be varied by varying its ballast, which is an exchange of mass with the environment, and its neutral depth is thereby controllable.
