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Pressure: Relative Density, Archimedes' Principle, Law of floatation and Atmospheric pressure

Density and Relative Density

Density is the measure of mass per unit of volume. The average density of an object equals it total mass divided by it's total volume. An object made from a comparatively dense material will have less volume that of an object of equal mass made from less dense substance. Perhaps highest density known is reached in neutron star matter. The singularity at the centre of the black hole, according to general relativity, does not have any volume, so it's volume is undefined.

The following table shall present the density of different substances:

Material  Density(kg/m3) 
Air (1 ATM 20 degree C) 1.20
Benzene 900
Blood 1600
Concrete 2000
Copper 8900
Silver 10500
Steel 7800
Neutron star 1018
White dwarf star 1010

Now talking about relative density it is the ratio of the density of a material to the relative density of a reference substance. It is usually measured with respect to water. It is commonly used in modern scientific terms to determine whether a certain substance is denser than reference substance or not. Relative density of any materials varies with the change in pressure and temperature of the material. The density of an object can be determined by dividing mass upon volume. The mass of your material is easy to calculate whereas volume can be ascertained in case of irregular materials. Following are various advantages if calculating through the relative density of an substance:

  • It helps in quantifying the buoyancy of fluids

  • It helps in determining density of unknown material

  • It helps in determining mineral content of rocks

  • It helps in identification of gems.

Also, the formulae of relative density is;

Relative density of a substance= Density of the substance/Density of pure water at 4oC

Note: Density of pure water at 4 Celsius= 1000kg/m3

The density of a liquid directly influences the upthrust. As the density of a liquid increases, the pressure given by the liquid increases. Upthrust is also increased in this condition. activity to proof this relation!

Archimedes' Principle

Archimedes Principle states that "The upward buoyant force that is exerted on a body immersed in fluid, whether partially or fully submerged, is equal to the weight of the body that the fluid displaces and acts in the upward direction at the centre of the mass of the displaced fluid." 

or, upthrust= weight of displaced fluid

Derivation of Archimedes' Principle

Experimental verification of Archimedes' Principle

Law of floatation

When a body floats in liquid, the weight of the liquid is displaced by its immersed part is equal to the total weight of the body. This is law of floatation.

So to put it simply, 

Weight of floating body= Weight of the liquid displaced by it's immersed part

Now, when a body is allowed to immerse in liquid, the following two forces act on it:

  1. the weight of the body acting vertically downward and,

  2. the upthrust on the body acting vertically upward.

Depending on the magnitude of these forces here are three things to know:

  1. a body sinks in a liquid only if the density of the body is greater than density of the liquid in which the body is kept

  2. a body just floats on a liquid when the density of the body is equal to the density of the liquid in which the body is kept

  3. a body floats keeping some parts of it out when its density is lesser than that of the liquid.

Atmospheric pressure


video for atmospheric pressure

Atmospheric pressure, also known as barometric pressure is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 101,325 Pa which is equivalent to 760 mm Hg. 

Atmospheric pressure is measured by using barometers. Mainly two types are used that being mercury barometer and aneroid barometer.diagram of each barometers.

Some instruments based on atmosphere pressure 

  1. Syringe

  2. Air pump

  3. Water pump

The following link has the explanation of full chapter in like 12 mins:https://www.youtube.com/watch?v=j6KIysJ1LT8

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