# Physics: Density, Specific gravity, Buoyancy, Upthrust and Archimedes principle

Density
This may be defined as the mass per unit volume of an object.

Density (d) = m/v

m = dv
v

Where, m represents mass and v represents volume.

Units in which Density can be expressed;
a) gram per cubic centimetres
b) kilograms per cubic metres

What is the density of water?
The density of water is 1 gram per cubic centimetres

What is the density of air?
The density of air is 1.2 kilograms per cubic metres. Material substances that are more dense than water will sink. Helium is less dense than air, so helium balloons rise. Oil floats on vinegar because it's density is less.

Specific Gravity
This may be defined as the the ratio of the density of a substance to the density of a reference substance.

In terms of mass, it is the ratio of mass of substance to that of a reference substance.

Specific gravity has no unit. It's value is highest in rocks like iron and other heavy metals and lowest in silica, water and so on.

Buoyancy
This may be defined as the ability of an object to float in water or air. It  is closely related to specific gravity. If a substance has specific gravity less than that of a fluid it will float on that fluid.

In physics, buoyancy or upthrust, is an upward force exerted by a fluid that opposes the weight of an immersed object.

Archimedes Principle
The buoyant force on a submerged object is equal to the weight of the fluid displaced. This principle is useful for determining the volume and therefore the density of an irregularly shaped object by measuring its mass in air and its effective mass when submerged in water (density = 1 gram per cubic centimeter)

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