Theory:

All liquids and gases are called fluids. A solid exerts pressure on the surface due to its weight. Similarly, fluids have weight, and they can also exert pressure on the base and walls of the container in which they are kept. The pressure exerted in any confined mass of fluid is transmitted undiminished in all directions.
 
Buoyancy:

Have you ever felt lighter after a swim from a pool? Have you ever drawn a bucket of water from a well and thought that the bucket is heavier when it is outside of the water surface? Have you ever wondered why a ship(Streamlined shape) made of iron and steel does not sink in seawater, but while the same amount of iron and steel in the form of a sheet or some other shape would sink? The concept of buoyancy can be used to answer these problems.
 
Let us understand the meaning of buoyancy by doing an activity.
  • Take an empty plastic water bottle. Close the mouth of the bottle with an airtight stopper or cap.
  • Put the bottle in a bucket containing water or any other liquid. You can see that the bottle floats.
  • Push the empty bottle into the water. You can feel an upward force. Try to push it further down by applying more force. You will find it difficult to push the bottle deeper and deeper.
  • This indicates that water exerts an upwards force. The upward force exerted by the water increases as the bottle is pushed deeper into the water till it is completely immersed.
  • Now, release the bottle. When the force acting on the bottle is removed, it bounces back to the surface.
Does the force due to the gravitational attraction of the earth act on this bottle? If so, why doesn’t the bottle stay immersed in water after it is released? How can you immerse the bottle in water?
 
The force due to gravity acts on the bottle in a downward direction. So the bottle should be pulled downwards. But the water exerts an upward force on the bottle. Therefore, the bottle is pushed upwards. We have learned that when two forces act in the opposite direction, the object will move in greater force. Consequently, it rises up because the upward force by the water is high.
 
To immerse the bottle completely in the liquid, the upward force on the bottle due to water should be balanced. This can be achieved by applying external force downwards. This force should be at least be equal to the difference between the upward force and the weight of the bottle.
 
The upward force exerted by the water or liquid on the bottle is known as upthrust or buoyant force. All objects experience a force of buoyancy when they are immersed wholly or partially in a fluid. The strength of this buoyant force depends on the density of the fluid.