Buoyancy and Archimedes’ principle

In short: A body in a liquid experiences an upward buoyant force exactly equal to the weight of the liquid it displaces. That is Archimedes’ principle.

In more detail: A body floats if its average density is lower than that of the liquid. It sinks if it is higher.

In detail

• Steel ships: The hull is hollow and displaces a lot of water. The ship’s average density is below that of water.
• Salt water: It has a higher density than fresh water and so carries better. In the Dead Sea you almost float.
• Hot-air balloon and airship: The same principle in air. Warm air is lighter than cold.
• Submarines: They change their density by flooding and emptying ballast tanks.
• Ice: It floats because it is less dense than water (aggregatzustaende).