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  • The ability to float is called buoyancy.
  • It is the measure of the upward force which is exerted by fluids on an object.
  • Liquids and gases do not have fixed shape and have the tendency to flow are called as FLUIDS.
  • Objects in fluid experience an upward buoyant force and thus result an object to float.
  • Buoyant force is the force that is acting on an object due to the surrounding fluid.
  • Buoyant force is also known as UPTHRUST

When we immerse the cube in the water we observe:

  • Downward force acts on the cube due to force of attraction by the earth.
  • Upward force acts on the cube due to water which gives an upthrust to the cube.
  • To keep an object immersed in water, the upthrust must be balanced and this can be achieved by applying external downward force.
  • When an object pushes water, the water also pushes the object with as much as force it can.
  • For object to float, the tendency of force of water must be more as compared to the force given by the object.
  • For an object to sink, the force of water is less as compared to the force exerted by the object on the downward direction.
  • The buoyant force is equal to the weight of water displaced.
  • The stone sinks in water and this is due to more downward pull as compared to upward force.
  • Flotation and sinking of object also depends on density and weight of the object.
  • Objects having less density than that of liquid floats and those having more density than that of liquid sinks in water.


  • In hot air balloon:  As air inside is heated, it becomes less dense than the surrounding and then the buoyant force pushes the hot air balloon upwards.
  • SUBMARINE: At the basement, the submarine have ballast tanks,  when the ballast tank are filled with sea water then the submarine sinks because the weight of submarine becomes more than the sea water. Moreover when the ballast tank is emptied with sea water the weight reduces thus the submarine rises up to the sea level again.


      It states that “when a body is immersed fully or partially in a fluid, it experience an upward force that is equal to the weight of the fluid displaced by it”.


Smriti Kanchan

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