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 Physics Formulae
Units of Measurements

Introduction: - Measurement of any physical quantity involves comparison with a certain basic, arbitrarily chosen, internationally accepted reference standard called unit.

Magnitude: - The number of times, a standard quantity is present in the physical quantity, is known as the ‘magnitude’ of the physical quantity.

Fundamental Quantity: - A physical quantity which is independent of any other quantity is a fundamental quantity.

Fundamental Unit: - The unit of a physical quantity which is independent of any other quantity is called a fundamental unit.

Complementary Fundamental Units: - Units of angle (degrees) and solid angle.

Derived Quantities: - The physical quantities which can be derived from other physical quantities are called derived quantities.

All other quantities except the fundamental quantities are derivable. The units of these quantities are also derivable from the fundamental units and are called derived units.

Derived Units: - The units of physical quantities which can be expressed in terms of fundamental units are called derived units.

For example, area, pressure, density and speed are derived quantities and their units – square meter, Pascal kilogram metre-3 and meter second-1 are derived from the fundamental units.

SYSTEMS OF UNITS

To measure the fundamental quantities length, mass and time there are three standardized systems of units. They are:

1. CGS (Metric)

2. FPS (British)

3. MKS System.

Their units are as shown in the table below:

 System Length Mass Time FPS CGS MKS Foot Centimeter Meter Pound Gram kilogram Second Second Second

C.G.S. System: In this system the unit of length is centimeter; the unit is mass of gram, and the unit of time is second.

F.P.S. System: In the F.P.S. system, the unit of length is foot, the unit of mass is pound and the unit of time is second.

M.K.S. System: In this system, the unit of length is meter, the unit of mass is kilogram and the unit of times is second.

Coherent System of Units: - A coherent system of units is a system based on a certain set of basic (or fundamental) units from which all derived units may be obtained by simple multiplication or division without introducing any numerical factors. S.I. system of units is a coherent system of units for all types of physical quantities.

S.I. Units: - S.I. unit is the short name of “System International de units “which means “International system of units”. It is an extension of M.K.S. system.

Derived S.I. Units