Physical Quantities and Measurements
Measurement of density
A body, when immersed in a liquid, experiences two forces—the weight of its own and the counteracting buoyant force. The figure shows the forces and their point of action.
These forces can give rise to the following situations as shown in the table below.
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CASE − I When W > FB |
CASE − II When W = FB |
CASE − III When W < FB |
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In this case, the body sinks to the bottom. |
In this case, the body floats below the surface. |
In this case, the body floats above the surface. |
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Law of Floatation: The weight of the solid floating in a fluid is equal to the weight of the fluid displaced by immersed part of the solid. |
The law of floatation can be represented as:
Volume of body × Density of body = Volume of displaced liquid × Density of liquid
Application of Law of floatation
- An iron ship floats easily
- If an iron nail is placed on the surface of water, it sinks while a ship made up of iron does not. This is because the density of iron nail is greater than that of water. Hence, the weight of the nail would be more than the upthrust of water on it due to which the nail sink in water. Whereas, the ship has large empty space in it which is occupied by air. This makes its volume large and average density less than that of water. Therefore, the weight of water displaced by the submerged part of the ship becomes equal to the total weight of the ship and hence it floats.
- An iceberg floats on water
- The density of ice is 0.917 g cm-3 and that of water is 1 g cm-3. Hence, big masses of ice, also called icebergs, float on water with their major part inside the water surface and only a small part above the water surface.
- Vol…
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