Work out the buoyant force on an object submerged in a fluid, based on Archimedes' principle.
Archimedes' principle states that an object submerged in a fluid experiences an upward force (the buoyant force, F) equal to the weight of the fluid it displaces. This force depends on the fluid's density (ρ), the displaced volume (V) and the acceleration of gravity (g = 9.81 m/s²).
This is why a steel ship can float, even though steel itself is far heavier than water: the hull is shaped hollow, so the ship displaces a large volume of water compared to its own weight. As long as the buoyant force is greater than or equal to the ship's weight, it floats.
Even though steel has a much higher density than water, a ship's hull is shaped hollow and filled with air. This means the ship displaces a large volume of water relative to its own weight. As long as the buoyant force from the displaced water is as large as the ship's weight, it floats.
Displaced volume is the volume of fluid that a submerged object "pushes aside" or replaces when it's lowered in. For an object that's fully underwater, the displaced volume equals the object's own volume.
Salt water has a higher density than fresh water (about 1025 kg/m³ versus 1000 kg/m³), and since buoyancy is proportional to fluid density, the buoyant force is greater in salt water. That's why it's easier to float in the ocean than in a freshwater pool.