Units and Measurements — Physics STD 11 Science — Question
Maharashtra BoardEnglish MediumSTD 11 SciencePhysicsUnits and Measurements4 Marks
Question
List the conventions followed while using $\text{SI}$ units.
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Answer
Following conventions should be followed while writing $\text{S.I.}$ units of physical quantities:
$1.$ Unit of every physical quantity should be represented by its symbol.
$2.$ Full name of a unit always starts with smaller letter even if it is named after a person, eg.: $1$ newton, $1$ joule, etc. But symbol for unit named after a person should be in capital letter, eg.: $N$ after scientist Newton, $J$ after scientist Joule, etc.
$3.$Symbols for units do not take plural form.
$4.$Symbols for units do not contain any full stops at the end of recommended letter.
$5.$The units of physical quantities in numerator and denominator should be written as one ratio. For example the $\text{SI}$ unit of acceleration is $m/s^2$ or m $s^{-2}$ but not $m/s/s.$
$6.$Use of combination of units and symbols for units is avoided when physical quantity is expressed by combination of two. For example, The unit $J/kg \ K$ is correct while $joule/kg \ K$ is not correct.
$7.$A prefix symbol is used before the symbol of the unit.
$a.$Prefix symbol and symbol of unit constitute a new symbol for the unit which can be raised to a positive or negative power of $10.$
For example,$1 ms = 1$ millisecond $= 10^{-3} s$
$1 \mu s = 1$ microsecond $= 10^{-6} s$
$1 ns = 1$ nanosecond $= 10^{-9} s$
$b.$ Use of double prefixes is avoided when single prefix is available
$c. \ 10^{-6} s = 1 \mu s$ and not $1 mms$
$10^{-9} s = 1 ns$ and not $1 m\mu s$
$8.$Space or hyphen must be introduced while indicating multiplication of two units e.g., $m/s$ should be written as $m s^{-1}$ or $m-s^{-1}.$
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