Science: Measurement in Science and Technology: SI Units and Derived Units (For CBSE, ICSE, IAS, NET, NRA 2022)

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Why Cannot the Parts of Human Body be Used for Accurate Measurement?

The results of the measurements vary from person to person because size of the unit is different for different person. For example, the units like a cubit or a foot would depend on the person making measurement. This created problems in trade between different countries and obviously in the day to day transactions.

SI Units and Derived Units

  • The International System of Units (SI, abbreviated from the French System international (d unites) is the modern form of the metric system and is the most widely used system of measurement.
  • The SI units for all other physical quantities such as area, density, velocity can be derived in terms of the base SI units and are called derived units.

Since,

Some Examples of Derived SI Units of the Commonly Used Physical Quantities

Some Examples of Derived SI Units of the Commonly Used Physical Quantities
Derived QuantityDimensionsName of UnitSymbol of
the Unit
areaLength × lengthsquare meterm2
volumeLength × length × lengthcubic meterm3
speed, velocitymeter per secondm s – 1
accelerationmeter per second squaredm s – 2
wavenumberreciprocal meterm – 1
densitykilogram per cubic meterkg m – 3
Workkilogram square meter
per square second

SI Prefixes

SI Prefixes
MultiplePrefixSymbolSub multiplePrefixSymbol
1024yottaY10 – 1decid
1021zettaZ10 – 2centic
1018exaE10 – 3millim
1015petaP10 – 6microm
1012teraT10 – 9nanon
109gigaG10 – 12picop
106megaM10 – 15femtof
103kilok10 – 18attoa
102hectoh10 – 21zeptoz
101decada10 – 24yoctoy

The rule is that the prefix is chosen in such a way that the resulting value of the physical quantity has a value between 0.1 and 1000. Let us illustrate it with examples , . Radius of Sun = 6.96 × 108 m = 696 × 106 m = 696 Mm (696 mega meter) , Alternatively = 6.96 × 108 m = 0.696 × 109 m = 0.696 Gm (0.696 giga meter) .

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