Hello Friends,

Today our topic is Practical units for measurement we have already studies all nine basic or fundamental quantities in SI system all the country in the world accepted this standard of units but why we study this practical units because in different location our old unit was using and we found that using this unit measurement was convenient and easy our practical life in SI system to express a unit we must know physics and mathematics for an example if we want to take milk we must know the cubic meter because we measure volume in meter but when we ask the milkman for milk he reply how much liter he does not understand cubic meter he know only liter and this is volume so it should be measure in meter but milkman does not understand meter cubic he has nothing to do with physics he has to sell milk and we want to buy milk so we made practical unit which we use in our day to day life.

Practical units measurement method |

When we load coal on truck we ask with loading man how much coal is loaded inside the truck he says 10070 kg again ask for next truck he replay 15090 kg this seems some long number compare is not easy so me made a practical unit ton which is use for this large mass how we converted 1000 kg for one ton other example we fill grain in sack customer comes and ask how much of cost seller told 15 rupees kg so for 95 kg he has to calculate for this purpose we made an another practical unit quintal and it is define as 100 kg equal to one quintal hence we fixed 100 kg in a sack so it become easy and convenient. if we want to measure a very very long length so we have to add so many thousand, lakh, crore meter if we want to measure distance form earth to sun, sun to other planet mercury, Mars, Jupiter etc if we use to measure in SI system it will be a very very big number so for that we made a practical unit for easy and convenient now question is how we made unit for this type of length measurement so answer is we made a unit distance from earth to Sun Astronomical unit and is denoted by A.U now this astronomical unit is distance from earth to sun is called one A.U now it is easy to compare the distance of other planet from this A.U unit now it is define as the mean distance between earth and sun one A.U why mean distance because earth rotate about sun its path is elliptical not circular so its mean distance is taken now we don’t know A.U we know SI unit meter so one A.U is equal to 1.496*10^11 meter and it is approx to 1.5*10^11 meter till date we were study about our planet only but when our astronomy developed then we started to study about stars now for star distance measurement from earth we made another practical unit why because in A.U measurement its number will be very big digit lakh and crore so we made another unit light year to measure the distance from earth to stars year is a unit of time but light year is a unit of length or distance first we made its definition and then we calculated in meter light travels with high speed we know 3*10^8 m/s that is in one second light travels 3*10^8 meter hence in one hour, one day, one month, one year and so on it will travel many many large distance hence for one year we allow light to travel the distance and that distance is called “light year” so it is define as one light year is length which is travel by light in one year. so when we see star in universe and say that star is 10 light year distance from earth its mean that 10 years before that light was emitted by star and today we are seeing hence how much it will be in meter it is simple calculation one light year = 3*10^8*60*60*24*365 meter that is equal to 9.46*10^15 meter now compare with A.U which one is big of course light year is big hence it is larger unit ten thousand times so use it for stars distance measurement now scientist made one another unit for distance measurement between stars to stars which is know as “parsec” with the help of angle taking two line of one degree inclination we subtends angle one minute (1′) = 1^0/60, one second (1″) = 1’/60 when we increase the length subtends angle will be small hence we know that angle = arc/radius now for making unit scientist fixed arc length one A.U because we know AU hence when one second (1″) angle subtends by AU called one “parsec” hence it is define as one parsec is the radius of a circle at the centre of which, an arc of the circle is 1 AU long subtends an angle of one second (1″) hence one parsec = 3.1*10^16 m hence parsec is largest unit for length or distance measurement.whereas we are not using this unit in our home because it is astronomical use for planet and stars but some unit we use in our home like foot, inch,cm, yard because we are using this unit from Britisher time so we have become some ideas about yard and feet one yard is equal to 3 feet it is strongly attached with Indian market whenever we want to buy scale it is of one foot 12 inch because we are making this scale from old time and till continue whenever we make room then say size of room 12 feet by 10 feet and we understand well the room size so foot has became practical unit similarly inch hence one inch = 0.0254 m or 2.54 cm one foot = 0.3048 m and one yard = 0.9144 m one mile = 1609 m commonly use in America for air flight we use nautical mile hence one nautical mile = 1852 m whatever object travel distance in air always express it distance in nautical mile or when we travel distance on earth surface express in mile, now when we measure very small particle then we made another small practical unit like when we study about material atom, molecules when we measured one molecule size it was equal to 10^-9 m so we given this name as nano hence one nano meter = 1nm = 10^-9 m we can see through naked eye up to 10^-4 m less than this like 10^-5, 10^-6 now we are not able to see through naked eye can’t distinguish water molecule then we use to see through microscope then our unit became one micron = 10^-6 after that we made more small for atom size unit is one Angstrom and it is equal to 10^-10 m use for atom size measurement after that again we search in nature for small particle 10^ -11 m, 10^-12 m, 10^-14 m not got any particle for this size and scientist search continue finally got 10^-15 m particle electron, proton, then made unit fermi and it is equal to 10^-15 m then why we not made 10^-20 m answer is simple we have not got any particle for this size till date hence smallest particle measure by fermi practical unit now i hope practical unit concept is cleared thanks for reading