Archimedes' principle Why do steel ships float? Sinking ships How do submarines float and sink? What about balloons?

You will be aware that some objects float in water and that others sink. When you swim you feel that you are lighter and more buoyant. The general concept that governs floating and sinking is called Archimedes' principle.

Archimedes' principle
An object immersed in a liquid experiences an upward force, called upthrust or buoyancy. This has been generally recognised for a long time. It was the Greek mathematician and philosopher Archimedes (287-212 BC) who first put it into a general principle. His statement, now known as Archimedes' principle, was that "when an object is immersed in a liquid the upthrust is equal to the weight of liquid displaced by the object". Thus, a rubber duck floats because its mass is equalled by the water it displaces before it physically goes under the surface. A solid lump of iron on the other hand will sink, because the water cannot displace the mass of the iron before the iron sinks.

Archimedes' principle.

Why do steel ships float?
It's obvious that a piece of steel will sink in water, yet ships made of steel float. There must be some other factor that comes into play to explain this. Applying Archimedes' principle to the problem the upthrust of the water must be greater than the mass of the ship, so somehow a great mass of water has to be displaced. This is done by shaping the hull of the ship in such a way that as the ship sinks into the water it displaces more and more liquid until a balance is reached between the mass of water displaced and mass of the ship. This general principle applies to any object made from a material that is more dense than the liquid it is in.

Sinking ships
In the past, some European ships sank when they entered the tropics for the first time. All was well when the cargo was put aboard in cold, salty waters, but then the ship sank when it reached warmer, less salty seas. The problem was that Archimedes' principle had not been taken into account. When the ship was first loaded it floated because cold, salty water has a relatively high density. This meant that less water had to be displaced to equal the mass of the ship. As the ship steamed to warmer, less salty waters, more water had to be displaced to maintain equilibrium. The ship dropped lower in the water, sometimes dropping below the waterline, and sinking. This was overcome by Samuel Plimsoll who marked his ships with what became known as the Plimsoll Line.

The Plimsoll Problem

How do submarines float and sink?
Using Archimedes' Principle, it is clear that a change in mass of an object affects how much liquid has to be displaced. In submarines, this is controlled by ballast tanks. When the tanks are empty, the submarine has less mass and it floats like a normal ship. As water is allowed into the tanks, the mass of the submarine increases, the downward gravitational force on the submarine increases and the submarine begins to sink. Careful balancing of the water ballast enables the craft to stay at any chosen depth. The swim bladders of fish work in a similar manner.

Submarines

What about balloons?

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