2.UNITS AND MEASUREMENT

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• For measurement of any quantity we need units.
• There are two types of units :-

1. Fundamentals Unit
2. Derived Units

Fundamentals Unit

• There are 7 fundamental or base units.

                Fundamentals quantities-units-symbolls  

1. Length -metre-m
2. Mass-kilogram-kg
3. Time-second-s
4. Electric Current-ampere-A
5. Thermodynamic Temperature-kelvin-K
6. Amount of substance-mole-mol
7. Luminous Intensity-candela-cd

• Besides of these seven base units there are two more units defined for:-

1. Plane angle
2. Solid angle

Plane angle

• Plane angle is the ratio of arc and radius.
• It is denoted by dθ.
• dθ=ds/r, here dθ is the plane angle

                                ds is the length of arc

                                r is the radius

• The unit of plane angle is radian and denoted by rad.

Solid angle

• Solid angle is the ratio of  Intercepted area of the spherical surface and square of its redius.
• It is denoted by dΩ.
• dΩ= dA/r²   here dΩ is the solid angle

                            dA is the Intercepted area of spherical surface

                             r is the radius

• The unit of Solid angle is steradian and denoted by sr.

Derived Units

• Units that derived from fundamentals unit is called derived units.

The international system of units

• In earlier times scientist of different country use different system of unit for measurements of quantities.
• The three most common Unit system are:-

1. CGS System:-  In CGS system the unit of length is centi-meter , unit of mass is gram and unit of time is second.
2. MKS System:- In MKS system the unit of length is metre , unit of mass is kilogram and unit of time is second.
3. FPS System:- In FPS system the unit of length is foot, unit of mass is pound and unit of time is second.

• For simplicity a system is developed which is internationally accepted and it is called Systeme Internationale d' Unites ( International system of units (SI)).