Mass is a basic property of matter. It does not depend on the temperature, pressure or location of the object in space. The SI unit of mass is kilogram (kg). It is defined by taking the fixed numerical value of the Plank Constant h to be 6.62607015 × $10^{–34}$ when expressed in the unit of Js which is equal to $kg.m^2.s^{–1}$, where the metre and the second are defined is terms of C and ∆νcs.
While dealing with atoms and molecules, the kilogram is an inconvenient unit. In this case, there is an important standard unit of mass, called the unified atomic mass unit (u), which has been established for expressing the mass of atoms as
1 unified atomic mass unit = 1u = (1/12) of the mass of an atom of carbon-12 isotope C-12 including the mass of electrons = 1.66 × $10^{–27}$ kg
Mass of commonly available objects can be determined by a common balance like the one used in a grocery shop. Large masses in the universe like planets, stars, etc., based on Newton’s law of gravitation can be measured by using gravitational method (See Chapter 8). For measurement of small masses of atomic/sub atomic particles etc., we make use of mass spectrograph in which radius of the trajectory is proportional to the mass of a charged particle moving in uniform electric and magnetic field.
Range of Masses
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| Table 1: Range and order of masses |
The masses of the objects, we come across in the universe, vary over a very wide range. These may vary from tiny mass of the order of $10^{-30}$ kg of an electron to the huge mass of about $10^{55}$ kg of the known universe. Table 1 gives the range and order of the typical masses of various objects.
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