# Gravitation

## 07 Pressure In Fluids

#### PRESSURE IN FLUIDS

• Fluids are substances which can flow easily.
• All the liquids and gases are fluids.
• A fluid, whether a liquid or a gas, exerts pressure in all directions – even upwards.
• Our body feels lighter while swimming in the pool.
• Also, the bucket used to draw water from the well feels lighter when fully immersed in water than when partially immersed.
• This is because the body experiences a force from the downward direction opposite to the direction of the gravitational pull, which is responsible for the decrease in its weight.
• Same is the reason that a ball of plastic floats in water rather than sinking to the base by its weight. Some objects such as a plastic bottle float over water, while others such as a needle sink.

#### BUOYANCY

• It is defined as an upward force exerted by a fluid that opposes the weight of an object immersed in a fluid.
• The pressure at the bottom of an object submerged in the fluid is greater than that at the top because the pressure in a fluid column increases with depth.
• The difference in pressure results in a net upward force on the object known as buoyancy.

#### DENSITY:

• The density of a material is defined as its mass per unit volume.
• It gives a measure of how tightly matter is packed together.
• In numerical terms, density is defined as:
• The S.I. unit of density is
$\mathrm{g}/{\mathrm{cm}}^{3}$

and C.G.S. unit of density is

$\mathrm{g}/{\mathrm{cm}}^{3}$

.

• The density of water is 1000 kg or at  C.
• If the density of a substance is more than the density of water, then the substance will be heavier then water and hence sink in water.
• If the density of a substance is less than the density of water, then the substance will be lighter than water and hence float in water.

Relative density:

• Relative density of a substance is defined as the ratio of density of the substance to the density of water at  C. Relative density is also known as specific gravity of the substance.
• Since relative density is a ratio of similar quantities, it has no unit.
• The relative density of a substance expresses the heaviness (or density) of the substance in comparison to water.
• For example, the relative density of iron is 7.8 . Now, by saying that the relative density of iron is 7.8 we mean that iron is 7.8 times as heavy as an equal volume of water.
• If relative density of a substance is less than one it floats in water and substances with relative density greater than 1 sink in water.

Buoyant Force/ Upthrust:

• Buoyant force is the upward force acting on an object immersed in a liquid.
• The upward force exerted by a liquid is also known as ‘upthrust’.
• The buoyant force is also known as upthrust.

Factors Affecting Buoyant Force:

There are two factors in which e magnitude of buoyant force acting on an object immersed in a liquid depends:

• Volume of object immersed in the liquid:

The buoyant force exerted by a liquid depends on the volume of the solid object immersed in the liquid. The upward buoyant force increases with the increase in the volume of solid object immersed inside the liquid. The magnitude of buoyant force acting on a solid object does not depend on the nature of the solid object.

• Density of the liquid:

The buoyant force exerted by a liquid depends on the density of the liquid in which the object is immersed. An object whose density is greater than that of the fluid in which it is submerged tends to sink. Even a very heavy material like an iron block floats in mercury because mercury exerts a very high buoyant force on iron block due to its very high density. Thus, if the object is either less dense than the liquid or is shaped appropriately (as in a boat), the force can keep the object afloat.

#### Principle of floatation

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