Density, Specific Gravity & Buoyancy

Density is mass divided by volume, kg/m³. Specific gravity is that density compared with water, so it is just a number: SG 0.72 means 720 kg/m³, and anything with an SG below 1 floats in water. Aviation fuel sits around 0.80, which is why fuel quantity in kilograms depends on the fuel temperature and its measured SG, not on volume alone.

Whether a body floats has nothing to do with how heavy it is, only with how its density compares to the fluid's. Archimedes' principle says the upthrust equals the weight of the fluid pushed aside. A floating body settles at exactly the depth where the displaced fluid weighs the same as the body — so a steel ship floats because its hull encloses enough air to bring its average density below that of water.

That balance gives a result you can read straight off the two numbers: the submerged fraction equals ρ_object ÷ ρ_fluid. Ice at 917 kg/m³ in seawater at about 1025 sits with roughly 90% of its bulk below the surface, which is the iceberg you have heard about. Push the object density past the fluid density and the upthrust can no longer match the weight, so it sinks — but it still feels lighter underwater by exactly the upthrust, which is its apparent weight.

• Try this: Float the block, then read the submerged percentage. How does it compare to object density ÷ fluid density?
• Try this: Keep the object at 900 and switch the fluid to a denser one. Does it ride higher or lower, and why?
• Try this: Set the object denser than the fluid. It sinks — but what is its apparent weight on the bottom compared with its true weight?