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Physics Theory : Density

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Density is defined as the mass per unit volume of a substance.
∴ Density = Mass / Volume
The SI unit of density is kilogram/meter3 (kg/m3).
Another commonly used unit (CGS system) for density is g/cm3.
1 kg/m3 = 1000 g/m3 = (1000/1000000) g/cm3 = 1/1000 g/cm3.
So, divide by 1000 to convert from kg/m3 to g/cm3, and multiply by 1000 to convert from g/cm3 to kg/m3.


Density of water is about 1 g/cm3 or 1000 kg/m3 at room temperature.
Density of sea water (about 1.04 g/cm3 or 1040 kg/m3) is more than that of fresh water because of the salt dissolved in it.


Density of air is about 0.001 g/cm3 or 1 kg/m3 at room temperature and atmospheric pressure.


Approximate densities (in g/cm3) of some substances are given below.

  • Tungsten (19.4)
  • Mercury (13.6)
  • Lead (11.4)
  • Aluminum (2.7)
  • Glass (2.2 to 2.7)
  • Cork (0.05 to 0.1)

When a body floats, it displaces an amount of fluid equal to its own weight.

Physics Quiz on Density.
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