# Buoyancy and Archimedes' Principle

## •    A fluid exerts pressure due to its weight. Buoyancy or buoyant force is the upward force exerted by any fluid upon a body immersed in it.The magnitude of upthrust increases with increase in volume of the body immersed in the fluid or in other words with the increase in the fluid displaced by the body and decreases with decrease in density of the fluid. •    When a body floats in a fluid, the weight of the whole body acting vertically downward, is equal to the upthrust produced by the fluid displaced by the immersed part of the body. The weight of a body immersed in a fluid will always be less than its actual weight, due to upthrust or buoyant force. •    If a solid is immersed in a fluid, then the following three situations take place: When the weight of the body is greater than then the upthrust: The body sinks in such situation. When the weight of the body is equal to the buoyant force: The body floats inside the fluid. When the weight of the body is lesser than the buoyant force: The body will float on the surface of the fluid. •    The relation between the buoyant force and the apparent weight of the body is determined using Archimedes’ principle. Archimedes’ principle states that when a body is immersed partially or wholly in a fluid, the upthrust on the body is equal to the weight of the fluid displaced by the body. This upthrust force is directly proportional to volume of the body immersed in the fluid and density of the fluid. •    Density and relative density are two important terms related to fluid pressure. Density is defined as the mass per unit volume of a substance Relative density is the ratio of the density of the substance to the density of water at 4oC. Relative density has no units. Determination of relative density of solids and liquids using overflow can (Eureka can) is a common experimental procedure.

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• Q1

State one important impact produced by buoyant force exerted by water.

Marks:1

An object appears to weigh less when submerged in water, than when it is in air.

• Q2

What is relative density? What is its unit?

Marks:1

Relative density of a substance is defined as the ratio of its density to that of water at 4oC. Since relative density is the ratio of two similar quantities, so it has no unit.

• Q3

What is the density of water at 4oC?

Marks:1

The density of water at 4oC is 1000 kgm-3.

• Q4

What is buoyant force? Give its unit.

Marks:1

The upward force acting on an object when immersed in a fluid is known as buoyant force. Its unit is same as that of force i.e. Newton.