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 upward force or upthrust ( or buoyant force) due to water. Like liquids, gases also exert upthrust on objects inside them.
Archimedes' Principle : This principle states that when a body is wholly or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid displaced.
When an object is immersed in a fluid,* two forces act on it:
(i) the weight of the object acting downward,
(ii) upthrust acting upward. It is due to upthrust that objects apparently weigh less when immersed in fluids.
An angler pulling a fish out of water experiences the sudden increase in the weight of the fish as soon as it is out of water.
It requires relatively less effort to lift a large boulder off the bottom of a river bed as long as the boulder is under water. Once the same boulder is out of the water, considerably greater effort is required to lift it.
The relative values of the weight and upthrust determine whether an object will sink in a liquid or float in it. If the weight of the immersed object is greater than the upthrust, the object will sink. If the weight is equal to the upthrust, the object remains at any level like a fish. If the upthrust is greater than the weight of the immersed object it will float to the surface.
It can easily be shown that an object will sink in a liquid if its density is more than that of the liquid. If the density of the object is less than that of the liquid, it will float on it.
● Law of Floatation :- When a block of wood is placed in it sinks until the equal to its own weight. When this happens the block weight of water floats. This example illustrates the law of floatation, which displaced states: A floating body displaces its own weight of the fluid in which it floats.
Archimedes' principle and law of floatation can explain several phenomena.
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 hollow and contains air and, therefore, its density is less than that of water.
A ship sinks in water to a level such that the weight of the displaced water equals its own weight. Since the density of sea water is more than that of river water, a ship sinks less in sea water. It is for this reason that a ship rises a little when it enters a sea from a river.
It is because of the higher density of sea water that it is easier to swim in the sea.
A submarine has large ballast tanks. When these tanks are filled with water the average density of the submarine becomes more than that of water and it can dive easily. When the submarine is ready to surface, compressed air is forced into the ballast tanks forcing the water out, thus reducing the density of the submarine which can then rise.
A solid chunk of iron will sink in the water but float in mercury because the density of iron is more than that of water but less than that of mercury.
A balloon filled with a light gas, such as hydrogen, rises because the average density of the balloon and the gas is less than that of air. The balloon cannot rise indefinitely because the density of the air decreases with increasing altitude. At a certain height, where the density of air is equal to the average density of the balloon, it ceases to rise and drifts sideways with the wind.
Ice, being less dense than water, floats in it with one tenth of its volume above the surface. When ice melts it contracts by as much of its volume as was above the surface and, therefore, the level of water remains unchanged.
