Matter occupies space. A particle of sand, mountain, the earth itself, all occupy space. A physicist has to measure space in distance that range from sub-atomic dimensions to those of the vast reaches of the universe. To get a precise idea about a physical phenomenon or problem, measurement is of basic importance to a physicist.
Need for Measurement:
Experience shows that our natural senses fail to make a comparative study of different objects. For example, the sun and the star in the sky appear very small in comparison with the earth but actually they are many times bigger than the earth.
Similarly, by just seeing, it is difficult to tell which one is bigger and heavier, a cricket ball or a hockey ball. Hence, to express our ideas in exact terms, measurement of physical quantities is important. Many of the discoveries in Physics have only been possible because of precise measurement.
Measurements in Physics: All measurements in Physics, even of such things as electric current are related to the three chosen fundamental quantities of length, mass and time.
U nits may be defined as a fixed part of a quantity by means of which comparison could be made of the magnitudes of the same quantity contained in different substances.
Metric Units in Common Use:
The prefixes used in the system are as under:
Mega means 10-2 Kilo means 10 Deci means 10 Centi means 10-2 Milli means 10can be expressed in terms of mass, length and time. The units of these quantities are called fundamental units and the quantities are called fundamental quantities.
All other units which have been created by the combination of one or more of the fundamental units are called derived units.
Systems of Units:
The following systems are commonly used and universally recognised.
(i) F.P.S. System:
In this system, the units of length, mass and time are foot, pound and second respectively.
(ii) C.G.S. System:
In this system, the units of length, mass and time are centimeter, gram and second respectively.
(iii) M.K.S. System:
In this system, the units of length, mass and time are metre, kilogram and second respectively.
S.I. Units (International System of Units): The eleventh General Conference of Weights and Measures has recommended a unified and systematically constituted system of fundamental and derived units for international use. In this system, the fundamental units are metre (m), kilogram (Kg) and second (s) respectively.
It is an instrument which is used by engineers and scientists to measure accurately much smaller lengths than millimeter. Screw-Gauge: It is another instrument which is used for measuring accurately the diameter of a thin wire or the thickness of a sheet of metal.