Physics
Buoyancy
Why a steel ship floats while a steel ball sinks: the water pushes back.
What you need first
In the swimming pool you feel almost weightless, and a huge container ship made of steel sits calmly in the harbour even though steel is much heavier than an equally large piece of water. Both have the same reason: water pushes up from below against everything that dips in. This force is called buoyancy, and in this chapter you will find out what it depends on.
The water pushes from below
Push a ball under water and you immediately feel it being pushed upwards. This upward force is called the buoyant force. It works against the that pulls the object down. You can even measure it: hang a stone on a and dip it into water, and the scale suddenly shows less. The difference is exactly the buoyant force.
The displaced water decides
Archimedes noticed it in his bathtub: whatever dips into water pushes water aside. Exactly this displaced water determines the buoyancy. The buoyant force is as big as the weight of the displaced water. As a rule of thumb: 1 litre of displaced water gives about 10 N of buoyancy. Dip an object in completely: if the buoyancy is then bigger than its weight, it rises and floats at the surface. If it is smaller, the object sinks. If both are equal, it hovers in the water. In short, what counts is whether an object is lighter than the equally large piece of water it pushes aside: a wooden cube is lighter than a water cube of the same size, a stone cube is heavier.
The comparison with the density of water (1.0 g/cm³) decides.
Why does a steel ship float?
A solid steel ball sinks because it displaces only a little water compared to its weight. A ship made of the same steel, however, is built hollow: its hull is full of air and pushes a huge amount of water aside. That makes the buoyancy as big as the weight of the whole ship, so it floats. It is not the material that decides, but the shape.
Hovering like a submarine
A submarine can change its weight: it lets water into its tanks to become heavier and sink, and pushes the water out again with air to rise. When weight and buoyancy are exactly equal, it hovers at depth. Divers do the same: with lead weights and an inflatable vest they tune their balance.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
In which direction does the buoyant force act?
A stone hangs on a spring scale. In air it shows 8 N, in water only 5 N. How big is the buoyant force in N?
Why does a ship made of steel float?
The buoyant force is exactly as big as the weight of the … water.
An object displaces 2 litres of water. 1 litre of displaced water gives about 10 N of buoyancy. How big is the buoyant force in N?
An object is completely under water. Match each behaviour with the condition that causes it.
Where this leads