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| Buoyancy and Archimedes' Principle |
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| It is a matter of common experience that bodies appear lighter when immersed in water or any other liquid. While bathing we notice that the mug of water suddenly appears heavier as soon as it comes above the water surface. Similarly, when a fish is pulled out of water, it appears to be heavier in air than inside the water. Now let us see why it is so. |
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| Objects appear to be less heavy in water or in any liquid because the liquid or water exerts an upward force on the objects immersed in it. Now by performing an experiment let us find out whether there is an apparent loss of weight when immersed in water. |
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| Take a stone and tie it to one end of the spring balance. Suspend the spring balance as shown in the figure. |
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| Experimental Set up to Prove Archimedes' Principle |
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| Note the reading on the spring balance. Let it be W1. Now, slowly dip the stone in the water in a container and note the reading on the spring balance. The reading shown on the spring balance goes on decreasing until it is completely immersed in water. The reading on the spring balance gives us the weight of the stone. Since the reading goes on decreasing, we can infer that the weight of the object is decreasing when it is lowered in water. The apparent loss of weight shows that a type of force is acting on the object in the upward direction thereby decreasing the weight. |
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| Thus the upward force acting on an object immersed in a liquid resulting in the apparent loss of weight of the object is called the buoyant force. |
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| The tendency of a liquid to exert an upward force on an object placed in it thereby making it float or rise is called buoyancy. |
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| Factors Affecting the Buoyant Force |
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| We know that when an iron nail is placed on the surface of water it sinks whereas ship made up of iron floats. This is because size or volume of the ship is more. |
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| Similarly when an iron nail and a cork of some mass is placed on water, the iron nail sinks because the density of iron nail is more than the density of water and whereas density of the cork is less than that of the water. Thus if the density of the liquid is more than the density of the material of the body then the body floats due to the buoyant force exerted by it and vice-versa. |
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| From the above examples we can infer that the buoyant force experienced by a body when submerged in a liquid depends on the volume of the body and the density of the liquid. |
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| Archimedes' studied the upthrust acting on a body, when it is partially or completely immersed in a fluid by performing several experiments and then stated the following principle known as the Archimedes' Principle. |
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| According to this principle, when a body is partially or wholly immersed in a fluid, it experiences an upthrust (buoyant force) equal to the weight of the liquid displaced. |
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| Take a clean and dry beaker and find it's mass (m) using a physical balance. Now find the weight of a stone by suspending it from a spring balance. Fill an Eureka can (Eureka can is a beaker having a spout near the top) with water filled till the spout. Place the beaker of mass 'm' under the spout. Gently lower the solid, suspended from spring balance, into the Eureka can, till the stone is completely immersed in water. When the stone is immersed in water it displaces a certain amount of water. The spring balance records lesser value thereby showing that the solid experiences an upthrust. The displaced water is collected in the beaker. Using the physical balance the mass of the water and beaker is determined. Let it be m1. |
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| If we compare the apparent loss of weight of the solid in water, with the amount of water displaced, it is found that they are equal. This experiment thus verifies Archimedes' Principle. |
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| It is used in designing ships and submarines. The lactometers and hydrometers used for measuring the purity of a sample of milk and for determining the density of the liquids are based on this principle. |
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