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Measurement of any physical quantity involves comparison with a certain basic arbitrarily chosen and widely accepted reference standard called unit. S.I units basic units 1. length-----metre 2. Mass--- kilogram 3. Time------ second 4. Electric current---- ampere 5. Thermodynamic temperature----- Kelvin 6. Luminous intensity---- candela 7. Amount of substance----- mole Supplementary Units 1. Plane angle --- radian 2. Solid angle ----- steradian IMPORTANT DERIVED UNITS 1. AREA --- LENGTH SQUARE----- m^2 2. VELOCITY----- DISPLACEMENT PER UNIT TIME----- ms^-1 3. FORCE----- MASS × ACCELERATION---- kgms^-2 GREATEST UNITS 1. 1 LIGHT YEAR =9.46 × 10^15 m 2. 1 parsec = 3.086 × 10^16 m =3.26 ly 3. 1 AU = 1.5 × 10^11m 4. 1. METERIC TONNE = 10^3 kg 5. 1 quintal = 10^2 kg