Units are necessary in physics to measure physical quantities and enable consistent comparisons. There are two types of units - fundamental units which measure fundamental quantities like length, mass, and time, and derived units which measure derived quantities like area, speed, and force. The International System of Units (SI) defines seven base units, including the meter, kilogram, and second. Units are also defined for other physical quantities using prefixes to indicate multiples or submultiples of the base units, enabling measurement of a wide range of quantities. Proper choice of units allows for convenient measurement depending on the magnitude of the physical quantity.

1. PHYSICS
UNITS
GYANRAJ CHAVAN

2. Why we need Units ?
To solve problems and to understand the basics of the physics it is very important to know what is
a physical quantity, types of physical quantities, what is a unit, what are the units of different
physical quantities, types of units, symbols of units.
 There is one and only branch of science which measures a physical quantity, that branch of science is
“Physics”. Measurements have an important role not only in physics but also in every branch of science and
everywhere in our day-to-day life.
 To measure physical quantities we need units. Let’s try to understand necessity of measurements and units
of measurement in Physics.
 The information about a physical quantity, by description of its external properties like color,
taste etc is incomplete without knowing its temperature, size (dimensions), which depends
on measurement i.e. without measurements it is impossible to know about the
external properties of any object. So, it becomes necessary to measure it.

3. What is UNIT?
To measure a Physical quantity like Length, Area , Force , Energy etc . We need a "STANDARD OF
MEASUREMENT" of same physical quantity . This standard of measurement is called the UNIT of
the physical quantity.
 Generally we can use any convenient unit to measure a physical quantity depending on how
much magnitude we are measuring or in which system of units we want to measure it.
 Def: A standard reference of the same physical quantity is essential to measure any
physical quantity. That standard which we use to measure a physical quantity is called unit.

4.  if we choose standards which are not consistent, and which cannot be
reproduced then errors and confusion in measurements will creep in.
To avoid such confusion, instead of taking any undefined reference as a
standard, well-defined and universal standards are used. Such a reference taken
a standard is generally called a well-defined unit (or) unit.
 Measurement of every physical quantity will have two parts a number (n)
followed by a unit (u).
 Therefore n × u = constant.
Standard Units

5. How do we explain ?
If the length of a table is 1.2 meters.
In this measurement number n= 1.2 and unit is u = meter.
→ length (L)= n1u1 = 1.2 meters
→ length (L)= n2u2 = 120 centimeters
→ length (L)= n3u3 = 1200 millimeters
From the above data we can understand that
 we can measure a physical quantity in different units. Whatever may be the unit it’s value is same.
→ L = n1u1 = n2u2 =n3u3
If the unit chosen is smaller, the multiple number will be greater.
 If the unit chosen is smaller, the multiple number will be greater.

6. Relation between Unit and Numerical number
→u1>u2>u3 n1<n2<n3
 The units(u) of a physical quantity will be reciprocal to the multiple (n)
nu= constant = n1u1 = n2u2 =n3u3
u α 1/n or n α 1/u
n1/n2 = u2/u1

7. Which Unit is convenient ?
we can use any convenient unit to measure a physical quantity depending on
 How much magnitude we are measuring
 In which system of units we want to measure it.
What kind of unit we should use?
The unit should be
 Accepted internationally.
 Reproduceable
 Invariable
 Easily available
 Consistent
 Large, if the physical quantity to be measured is a big .
Ex : To measure larger lengths we use units like Km, mt etc, to measure large
magnitude of time we use units like hour , day ,week, month , year etc.
 Small, if the physical quantity to be measured is small.
Ex: To measure small time we use units like millisecond, microsecond etc.
To measure small lengths we use units like millimeter, centimeter etc.

8. We can broadly divide the physical quantities in to two types
i)Fundamental Physical quantities ii)Derived physical quantities.
Fundamental physical quantities: A physical quantity which can exist
independently is called Fundamental physical quantity.
Ex: Length, mass and time etc.
Derived physical quantities: A physical quantity which can not exist independently is called
derived physical quantity. (Or) A physical quantity which is dependent or derived from any
other physical quantity is called derived physical quantity.
Ex : Area, volume, density, speed, acceleration, force, energy etc.
Types of Physical Quantities

9. Types of Units
Like the physical quantities we can divide the units in to two types.
I)Fundamental units ii)derived units.
Fundamental units : The units of fundamental physical quantities are called fundamental
units, (or) The units which are independent or cannot derived from any other unit is called
fundamental unit.
Ex: Every unit of length is fundamental unit (irrespective of the system to which it
belongs);millimeter, centimeter, meter, kilometer etc.
Every unit of time is a fundamental unit ; microsecond, millisecond, second, minute, hour,
day etc.
Derived units: The units of derived physical quantities are called derived units. Units of
area, volume, speed, density, energy etc. are derived units.
Ex: Every unit of speed is a derived unit ; m/sec, cm/sec, km/hr etc.
Every unit of density is a derived unit; kg/m³, gr/cm³ etc.
Every unit of acceleration is a derived unit; m/sec², cm/sec², km/hr² etc.

10. Systems of Units
To measure the fundamental physical quantities Length, Mass and time we have three
systems of units, they are
i) C.G.S System (Metric system)
ii)F.P.S System (British system) and
iii)M.K.S System.
In all these three systems only three physical quantities length, mass and time are
considered to be fundamental quantities.

11. Systems, Fundamental quantities and their Units
System Length Mass Time
C.G.S System Centimeter Gram Second
F.P.S System Foot Pound Second
M.K.S System Meter Kilo-gram Second

12. Multiples of Units
But, in systems International (S.I) system there are seven fundamental physical quantities.
Which are i)Length ii)Mass iii)Time iv)Electric current v) Thermo-dynamic temperature
vi)Luminous intensity vii)Quantity of substance.
In addition to these two more quantities were added as supplementary physical quantities.
They are i)Plane angle ii)Solid angle.

13. S.I System of Units
S.I System
Fundamental Quantity Unit
Length meter
Mass kilogram
Time second
Electric current ampere
Thermo-dynamic temperature kelvin
Luminous intensity candela
Quantity of substance mole
supplementary quantity Unit
Plane angle radian
Solid angle Steradian

14. Multiples of Units
Depending upon the magnitudes of physical quantities we measure, we have to use different
multiplication factors suitable for that particular case. Here let us see some widely used
multiplication factors.
Multiplication Factor Prefix symbol
101 deca da
102 hecta h
103 kilo k
106 mega M
109 giga G
1012 tera T
1015 peta P

15. Sub-Multiples of Units
Multiplication Factor Prefix symbol
10-1 deci d
10-2 centi c
10-3 milli m
10-6 micro µ
10-9 nano n
10-12 pico p
10-15 femto f

16. Special Units of Length
 Micron (µ) = 10-6 cm = 10-4 mm
 Angstrom (A) = 10-10 m = 10-8 cm
 Fermi = 10-15m = 10-13 cm
 Astronomical Unit (A.U) = 1.5 × 1011 m = 1.5 × 1013 cm
 X ray unit (X.U) = 10-13 m ( To measure wane length of X-Rays
 Light year = Distance traveled by light in one year= 9.5×1015m =9.5×1012km
 parsec = 3.26 light years =3.1×1016 m
 1 agate(typography) = 0.07 inch = 1.8 cm
 1 cable’s length = 720 feet = 219.46 meters
 1 chain (engineer’s) = 100 feet = 30.48 meters
 1 chain (Gunter’s or surveyor’s) = 66 feet = 20.12 meters
 1 cubit = 18 inches = 45.72 centimeter
 1 degree (geographical) = 69.05 miles = 111.12 kilometers

17. Ø 1 decameter = 10 meters = 32.81 feet
Ø 1 fathom = 6 feet = 1.83 meters
Ø 1 hand = 4 inches = 10.16 centimeters
Ø 1 league = 3 miles = 4.83 kilometers
Ø 1 link (engineer’s) = 1 foot = 0.31 meter
Ø 1 link (gunter’s or surveyor’s ) = 7.92 inches = 10.16 centimeters
Ø 1 perch or pole = 16.5 feet = 5 meters
Ø 1 point (typography ) = 0.1 inch = 0.35 millimeter
Special Units of Length

18. Special Units of Time
 Solar day def: The time taken by earth to complete one rotation about its own axis with
respect to sun is called solar day. (Average value for all the days of one year is Mean
solar day).
 Sidereal day : It is 4.1min shorter than Mean solar day .
 sidereal year :365.26 Mean solar day d ) Solar year = 365.24 Mean solar day
 Lear year = The year in which February month has 29 days is called leap year. It is
divisible by 4.
 Lunar month :Time taken by moon to complete one rotation around earth is lunar
month = 27.3 days.

19. Special Units of Mass
 Atomic mass Unit ( a.m.u) : = 1/12 of mass of
C12 atom = 1.67×10-24gr = 1.67×10-27kg.
 1 assay ton = 29.167 grams = 1.03 ounces
 1 carat = 200 milligrams = 3.09 grains
 1 decigram = 100 milligrams = 1.54 grains
 1 decagram = 10 grams = 0.35 ounce
 1 pound = 0.45 kilograms

20. Special Units of Pressure
 Atmosphere =760 mmHg = 76×13.6×980 dyne/ cm2= 1.013×106 dyne/cm2
= 1.013× 105 N/m2 or Pascal
 Bar = 750 mmHg = 75×13.6×980 dyne/ cm2 = 0.99×106 dyne/cm2 = 0.99×
105 N/m2 or Pascal
 Torr =1 mm Hg = 0.1×13.6×980 dyne/ cm2 = 1333 dyne/cm2 = 133 N/m2 or
Pascal

21. Special Units of Area
Barn: this is unit of area, it is used to measure cross section of nuclei.
Barn = 10-28 m2
1 acre = 4047 sq meters = 4840 sq yards = 43560 sq ft
1 are = 100 square meters = 1076.39 sq feet
1 square(building) = 100 sq feet = 9.29 sq meters
1 sq link = 62.73 sq inches = 404.69 sq cm
1 township = 36 sq miles = 93.24 sq kms

22. Special Units of Speed
light wind = 7 miles/hr
light breeze = 11 miles/hr
gentle breeze = 16 miles/hr
moderate breeze = 20 miles/hr
fresh breeze = 25 miles/hr
strong breeze = 30 miles/hr
moderate gale = 35 miles/hr
fresh gale =45 miles/hr
strong gale = 50 miles/hr
whole gale = 60 miles/hr
storm = 70 miles/hr
hurricane = 80 miles/hr

23. Thank You