All objects have a mass.
The greater the mass, the greater the weight.
Weight is the gravitational force that pulls an object downwards.
But if we plunge an object in water, its weight seems less, as if the object had become lighter.
In fact, its mass, and therefore its weight remain the same.
However, a new force counteracts the weight.
This force is called the buoyant force or simply buoyancy. The object experiences the combined action of both forces.
If the buoyant force is less than the weight, the object falls, it sinks.
Inversely, if the buoyant force is greater than the object’s weight, it goes up.
The object floats when these two forces are balanced.
In any case, buoyancy is oriented upwards.
Would it be possible to increase this force enough to make a heavy object float?
The answer is « yes ! »
The Greek scholar Archimedes found the solution.
“A body immersed in a fluid whether fully or partially immersed, is equal to the weight of the fluid that the body displaces”
Let’s consider an example. When an object sinks in water, it displaces water.
The volume of water displaced is equal to the immersed volume of the object.
The bigger the object, the more water is displaced.
Archimedes asserted that the weight of the volume of water displaced is exactly equal to the buoyant force except that this force goes upward.
For buoyancy, the shape is as important as the mass
A compact mass displaces very little water, whereas a ship, given its shape, displaces a lot of water.
The greater the immersed volume, the greater the intensity of the buoyant force.
Thus, a light object can sink whereas a very heavy object can float.