Some bodies sink in water while some others float in it. On the other hand the same bodies which float in one kind of liquid sink in another kind. A piece of iron sinks in water, while a large ship made of iron floats. A swimmer feels lighter when he is inside water. These phenomena can be explained with the concept of buoyancy.
Fluids: Any substance which can flow is called fluid. It includes liquids and gases.
Thrust: The force with which a body tends to push the other when the two are brought in contact with each other is called thrust.
Buoyancy: The upward thrust which a body experiences when it is immersed partly or completely in a fluid is called buoyancy.
The effect of buoyancy is more obvious in case of liquid, then gas because gases are much lighter than liquids.
Experiment to confirm the existence of buoyancy:
We take a stone of mass 7 lb and suspend it in air by using a thread from a weighing scale. Now we immerse the stone in a beaker of water while it is suspended from the scale. It is seen that the weight of the stone has come down to 4 lb thereby losing 3 lb as a result of buoyancy. This confirms that an upward force exists which reduces the weight of the body.
Now the apparent loss of the weight of the body is also equal to the fluid displaced by the body. The displaced fluid of equal weight is shown collected in a container outside. The measurement of the apparent loss of weight was given by a scientist named Archimedes and is known as Archimedes principle.
Archimedes principle: It states that when a body is immersed partly or completely in a fluid the body appears to lose a part of its weight which is equal to the weight of the fluid displaced by the body.