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Why do ships float?

The buoyancy force acting on a ship If you dropped a small stone into the water, it would sink immediately. But if a block of steel weighing thousands of tonnes were made into a ship, it would float. Any object placed in water experiences two forces: gravity, which pulls it down because of its weight, and the buoyancy force, or upthrust, which pushes it up. When these forces are balanced, the object floats. The buoyancy force of a stone is much smaller than its gravity force, which is why the stone sinks (although the buoyancy force makes the stone sink slower than it would do through the air). A ship also experiences these two forces. But because of its body design, which contains a lot of air, it displaces (pushes aside) enough water so that the buoyancy force is equal to its gravity force. That is why a ship floats.

Markings, known as load lines, on a ship’s hull indicating displacement (the weight of water it pushes aside, and therefore the...Read More >>Markings, known as load lines, on a ship’s hull indicating displacement (the weight of water it pushes aside, and therefore the total weight of the ship together with its load). The more heavily loaded a ship is, the lower in the water it sits.

In fact, a ship does not float on the surface of the water; it sits in the water with part of its hull below the surface. The heavier the load, the deeper it sits. Because the pressure of water increases with depth, the deeper into the water the ship sits (without actually submerging completely), the more buoyancy force is created. So if a ship weighs 1000 tonnes, it will sink into the water until it has displaced 1000 tonnes of water. Provided it displaces 1000 tonnes before it is completely submerged, it still floats. But if the ship weighs more than the total volume of water it displaces, it will sink.

The person who first worked out the answer to why ships float was the Greek mathematician Archimedes in around 250 BC. His famous law became known as Archimedes' Principle: when something is resting in or on water, it feels an upward force equal to the weight of the water that it displaces.