Principle of floatation states that a body will float in a liquid if the weight of the body is equal to the weight of the liquid displaced by it.
When a body is placed in a liquid, then the two forces act on it:
- The upthrust/buoyancy which acts vertically upward due to liquid.
- The weight of the body (mg), acting vertically downward.
Weight of body = W
Density of body = ρ
Weight of liquid displaced = W’
Density of liquid = ρ’
W = Vρg and W’ = Vρ’g
Then we have following three cases :
- When weight of object is greater than upthrust or buoyancy,
W > W’
Vρg > Vρ’g
ρ > ρ’
Hence, a body completely sinks in liquid if density of body is greater than density of liquid.
- When weight of object is equal to upthrust or buoyancy,
W = W’
Vρg = Vρ’g
ρ = ρ’
Hence, a body immerses completely in liquid but remains floating in liquid if its density is greater than density of liquid.
- When weight of object is less than upthrust or buoyancy,
W < W’
Vρg =< Vρ’g
ρ < ρ’
Hence, a body floats in liquid if its density is greater than density of liquid. The portion of the body that lies inside the liquid displaces the liquid equal to its own weight.
Application of principle of floatation
- Ships: A ship is specially shaped body which floats in water. The ship displaces water whose weight is equal to the weight of the ship. If cargo and passengers are added in the ship, the ship sinks dipper and displaces additional water whose weight is equal to weight of the cargo and passengers. In this way, the ships always remain floating on water.
- Submarines: A submarine is a ship which can float as well as sink in water. The submarines are provided with huge tanks called ballast tanks. When the submarine has to dive, the ballast tanks are filled with water the weight of the submarine increases. Then, the submarine dives. To return to the surface, the water is taken out of the ballast tanks with the help of pumps. The buoyancy becomes greater than the weight of the submarine. As a result, the submarine comes up to the surface.
- Icebergs : The icebergs are the large mass of ice in sea or ocean. These masses remain floating in water according to the law of floatation since they have less density than that of water.
- Balloons : Weight of hydrogen gas is much less than that of water. When Hydrogen is filled up in balloon, the weight of air displaced by balloon becomes greater than the weight of balloon and its contents. As a result, the upthrust in balloon becomes greater than its weight and balloon is pushed upwards. Hence balloon rises up. When it rises up continuously, density of air goes on decreasing. At certain height, upthrust in balloon becomes equal to weight of balloon and its components and it stops to rise up as according to principle of floatation.
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