All the quantities in terms of which
laws of physics are described and whose
measurement is essential are called physical
quantities like mass, length, time, light,
current electricity, temperature, etc.
UNIT
The fixed part of a Physical quantity by
dint of which comparision could be made
of the magnitudes of the same quantity
contained in different substances is termed
as a unit.
Systems of units Length Mass Time
1. C.G.S. System Centimetre Gram Second
2. F.P.S. System Foot Pound Second
3. M.K.S. System Metre Kilogram Second
The measurement of a physical quantity
requires a unit and the comparision of unit
to the quantity & the number of the units
obtained gives the measured value of the
quantity e.g. when we say that the length of
a rod is 5 metres it means that the length of
the rod is 5 times the unit of length, metre.
FUNDAMENTAL UNITS
(i) Metre (m) : 1 metre is equal to the
length of the path travelled by light in
vacuum in 1/2 99792458 of a second where
speed of light is 299792458 m/s. Recently,
the definition of 1 metre of length is realized
by using the iodine stabilized helium neon
lasers.
(ii) Kilogram (kg) : A standard block of
Platinum Iridium alloy preserved in the
International Bureau of Weights & Measures
at Sevres, near Paris, France is used as a
prototype of unit of mass and one kilogram
is equal to the mass of this alloy.
(iii) Second (s) : The definition of second
is based on an atomic clock which works
on energy radiation from an isotope of
caesium (Cs-133). One second is equal to
the duration of 9, 192, 631, 770 periods of
International System of Units (S.I.)
The system is a modification over the MKS system
and hence, is known as rationalised MKS System.
There are seven fundamental and two supplemen-
tary units in S. I. System :
Physical Fundamental Symbol
1. Mass Kilogram kg
2. Length Metre m
3. Time Second s
4. Tempertature Kelvin K
5. Electric-current Ampere A
6. Luminous intensity Candela Cd
7. Quantity of matter Mole mol
Supplementary Supplementary Symbol
Physical quantity Unit
1. Plane angle Radian rad.
2. Solid angle Steradian sr
The units of three quantities viz mass,
length and time are called fundamental
units because all physical quatities can be
expressed in terms of mass, length and
time and remaining all units are called
derived units.
radiation corresponding to unperturbed
transition between the two specific energy
levels of the ground gaseous state of Cs-
133 isotope. The caesium atoms in the
atomic clock act like a pendulum in a
pendulum clock. The atomic clock gives
the most accurate time with an error of 1
second only in 5000 years.
IMPORTANT PRACTICAL UNITS
For Lengths : (i) Astronomical Unit
(A.U) : It is equal to the distance between
the centre of the earth and the centre of the
sun. One A.U. = 1.5 × 1011m.
(ii) Light Year (ly) : It is equal to the
distance travelled by light in vacuum in
one year. 1 light year = 9.46 × 1015m.
■ ■ ■
(iii) Par sec. (Parallactic second) : 1
Par sec = 3.1 × 10¹⁶m = 3.26 ly
(iv) 1 micrometre or 1 micron = 10–6m
(v) 1 nanometre (1nm) = 10–9m
(vi) 1 Angstrom (1A°) = 10–10m
For Masses : (i) 1 tonne or 1 metric ton
= 1000 kg
(ii) 1 pound (lb) = 0.4536 kg.
(iii) The largest unit of mass is Chandra
Shekher Limit (C.S.L) 1 C.S.L. = 1.4 times
the mass of the Sun.
(iv) For small masses atomic mass unit
(a.m.u.) is used. 1 a.m.u = 1.6 × 10–27kg.
For Area : (i) 1 acre = 4047 m2
.
(ii) 1 hectare = 10⁴
m2
.
