Measurements

Physics can also be defined as the branch of science dealing with the study of properties of materials. To understand the properties of materials, measurement of physical quantities such as length, mass, time etc., are involved. The uniqueness of physics lies in the measurement of these physical quantities.


 Fundamental quantities and derived quantities Physical quantities can be classified into two namely, fundamental quantities and derived quantities. Fundamental quantities are quantities which cannot be expressed in terms of any other physical quantity. For example, quantities like length, mass, time, temperature are fundamental quantities. Quantities that can be expressed in terms of fundamental quantities are called derived quantities. Area, volume, density etc. are examples for derived quantities.



To measure a quantity, we always compare it with some reference
standard. To say that a rope is 10 metres long is to say that it is 10
times as long as an object whose length is defined as 1 metre. Such a
standard is called a unit of the quantity.
Therefore, unit of a physical quantity is defined as the established
standard used for comparison of the given physical quantity.
The units in which the fundamental quantities are measured are
called fundamental units and the units used to measure derived quantities
are called derived units.



 In earlier days, many system of units were followed to measure physical quantities. The British system of foot−pound−second or fps system, the Gaussian system of centimetre − gram − second or cgs system, the metre−kilogram − second or the mks system were the three systems commonly followed. To bring uniformity, the General Conference on Weights and Measures in the year 1960, accepted the SI system of units. This system is essentially a modification over mks system and is, therefore rationalised mksA (metre kilogram second ampere) system. This rationalisation was essential to obtain the units of all the physical quantities in physics. In the SI system of units there are seven fundamental quantities and two supplementary quantities.

The SI system is logically far superior to all other systems. The SI units have certain special features which make them more convenient in practice. Permanence and reproduceability are the two important characteristics of any unit standard. The SI standards do not vary with time as they are based on the properties of atoms. Further SI system of units are coherent system of units, in which the units of derived quantities are obtained as multiples or submultiples of certain basic units. Table 1.2 lists some of the derived quantities and their units.

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