Fluid statics studies fluids at rest, and it explains buoyancy through a genuinely elegant principle: an object submerged, even partially, in a fluid experiences an upward force exactly equal to the weight of the fluid that object actually displaces. Fluid dynamics studies fluids in motion, covering how liquids and gases flow and exert force as they move. Together, these two branches explain a real, intuitive puzzle: a massive steel ship floats, because its hollow, spread-out shape displaces far more water than the ship itself weighs, while a small, solid steel coin made of the exact same material sinks straight to the bottom.
Buoyancy depends on displaced water's weight, not on the object's own material
The upward buoyant force on any submerged object exactly equals the weight of the fluid it displaces, regardless of what the object itself is actually made of, which means an object floats if that buoyant force is at least as large as the object's own weight, and sinks if it isn't. A solid steel coin displaces only a small amount of water, exactly its own small solid volume, nowhere near enough to generate a buoyant force matching steel's own considerable weight, so it sinks. A steel ship, hollow and spread across a much larger volume, displaces a genuinely enormous amount of water, easily enough to generate a buoyant force that matches or exceeds the ship's total weight, steel hull, cargo, everything included.
Shape, not material, is what actually decides whether an object floats
This is exactly why the same material, steel, can either float or sink depending entirely on how it's shaped: what matters for floating isn't the material's own density in isolation, but the average density of the object as a whole, hull plus the air-filled space inside it, compared to the surrounding water's density. A ship's hull is deliberately shaped to enclose a large volume of air, keeping the vessel's overall average density below water's, and it's precisely that shape-driven displacement, not any special property of steel itself, that keeps a genuinely massive ship afloat.
What we're still unsure about
That buoyant force exactly equals the weight of fluid displaced, and that an object's overall shape, not its raw material density alone, determines whether it floats, is extremely well established, confirmed physics dating back over two thousand years to Archimedes' original principle. What's more genuinely a matter of ongoing engineering complexity is exactly how a ship's real, complicated hull shape actually behaves once fluid dynamics, waves, currents, the water's own motion, is added on top of the simpler static buoyancy picture, since a ship's stability in moving water depends on considerably more than the basic displacement calculation alone, and naval architects continue to rely on detailed fluid dynamics modelling and testing to predict exactly how a given hull design will actually behave once real, moving water is involved.
This sits inside Fluid Statics & Dynamics, one of eight topics in Mechanics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.