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Buoyancy of the Liquid (Upthrust)

If a block of wood is held below the surface of water and then released, it immediately rises to the surface. The block rises because it experiences an upthrust (or buoyant force) due to water. All fluids exert an upthrust. When a body is immersed in a fluid, it experiences an upthrust or upward force. This upthrust exerted by a liquid on a body immersed in it is known as buoyancy of the liquid.

Archimedes Principle
 
According to Archimedes principle, when a body is wholly or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid displaced by it. In other words, a body immersed in a fluid loses its weight which is equal to the weight of the fluid displaced by it.
 
Law of Floatation
 
According to the law of floatation, a body floats in a liquid if the weight of the liquid displaced by the immersed portion of the body is equal to the weight of the body.
 
  • An iron nail sinks in water, whereas a ship made of iron and steel floats. This is due to the fact that a ship is hallow and contains air and therefore its density is less than that of water.
  • A ship sinks less in sea water as the density of sea water is more. It is for this reason a ship rises a little when it enters a sea from a river.
  • A balloon filled with a light gas, such as hydrogen, rises because the average density of the balloon and the gas in it is less than that of air.
  • Ice, being less dense than water, floats in it with one-tenth of its volume above the surface.




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