Unit and Measurements

What is Unit

Measurement of any physical quantity involves comparison with a certain basic arbitrarily chosen, internationally accepted reference standard known as unit. A Unit should be - well defined, easy to reproduce, easy to compare, internationally accepted and independent of changes in physical conditions.

Units are of two types :
1. Fundamental Unit
2. Derived Unit

Fundamental Unit

The physical quantities which do not depend upon other quantity are known as fundamental quantities and their units are known as fundamental units.

There are seven fundamental S.I. units given below :

Physical quantity Name of the unit Symbol
Length meter m
Mass kilogram kg
Time second S
Electric current Ampere A
Temperature Kelvin K
Amount of substance Mole Mol
Luminous intensity Candela Cd

Besides these seven fundamental units, two supplementary units are also defined, viz., radian (rad) for plane angle and steradian (sr) for solid angle.

Derived Unit

All the units which are defined/expressed in terms of fundamental units are called derived units.

Some practical units of length, mass and time
Length Mass Time
light year = 9.46 x 1015 m (distance that light travels with velocity of 3 x 108 m/s in 1 year) 1 tonne or 1 metric ton = 103 kg Solar day : It is the time taken by earth to complete one rotation about its axis w.r.t. then sun. 1 year= 365.25 solar days.
1 astronomical unit = 1 AU = 1.496 x 10 11 m. (average distance of the sun from the earth) 1 quintal = 102 kg Lunar month : It is the time taken by moon to complete one revolution around the earth in its orbit. 1 lunar month = 27.3 days
1 parsec = 3.08 x 1016 m (parsec is the distance at which average radius of earth’s orbit subtends an angle of 1 arc second) 1 pound = 0.4536 kg Solar year : It is the time taken by the earth to complete one revolution around the sun in its orbit.
1 micron = 10-6 m 1 atomic mass unit = 1 amu or 1 u = 1.66 x 1027 kg Tropical year : It is that year in which solar eclipse occurs.
1 angstrom = 10-10 m Leap year : It is that year in which solar eclipse occurs.
1 fermit = 1 f = 10-15 m 1 solar day = 86400 s
Prefixes used in metric system
Prefix Symbol Multiplier
deci d 10-1
centi c 10-2
mili m 10-3
micro µ 10-6
nano n 10-9
pico P 10-12
femto f 10-15
atto a 10-18
zepto z 10-21
yocto y 10-24
deca da 101
hecto 102
kilo k 103
mega M 106
giga G 109
tera T 1012
peta P 1015
exa E 1018
zatta Z 1021
yotta Y 1024