CHAPTER 1 :UNITS AND MEASUREMENTS
 WHAT
IS A PHYSICAL QUANTITY? GIVE EXAMPLES?
Any thing that can be measured is
a physical quantity. Example : length,
mass, time
Velocity, density
 What
is a Unit? Give examples?
Units are certain reference
standards used for making measurements.
Example: m/s, kg/ m^{3}, Km, Kg
 What
are fundamental quantities and fundamental Units?
The physical quantities mass, length and time are known as fundamental
quantities.
They are called so because, Units
of all other quantities in mechanics can be derived from fundamental
units. Also they can not be sub divided
in to smaller units
Units of fundamental quantities
are known as fundamental units.
(meter , kilogram, second )
 What
are derived quantities and derived units?
Physical
quantities that can be expressed in terms of fundamental quantities are
known as derived quantities.
(example :
m/s , m/s^{2}, m^{3 })
Note: Two parts for measured result:
Any measurement has two parts
 Its
magnitude
 Its
Unit
If we decrease the size of the
unit, the numerical; value will increase.
1 Km = 1000 m
Here Km is a large unit of length,
while magnitude is 1
But, meter is a small unit of
length and its magnitude is 1000
Smaller the size of the unit, greater is its numerical value.
 What
are the characteristics of a unit , that is chosen for making measurement ?
The unit that is chosen for making
a measurement should have the following properties:
 It
must be well defined.
 It
should be of suitable size
 It
should be accepted internationally
 It
should not change with change in temperature, pressure ,etc
 What
are the different system of units ?
CGS – Centimeter, gram , second
FPS – Foot, Pound, Second
MKS – Meter, Kilogram, second
 What are the physical quantities and units in SI system?
7 Basis physical quantities
of SI unit system are
Length

meter

M

Mass

Kilogram

Kg

Time

Second

S

Temperature

Kelvin

K

Electric current

Ampere

A

Luminous intensity

Candela

Cd

Quantity of matter

Mole

mol

2 Supplementary quantities of SI
system are
Plane angle

Radian

rad

Solid angle

Steradians

sr

 Define
1 meter in 3 ways ?
1)
It
is 1 / (10
^{7}) th of the distance from north pole to equator of earth

1 meter  old definition 
2) 1
meter is the distance between two lines marked on a PlatinumIridium rod
Kept at a constant temperature of
273.16 K and at 1 bar pressure in the
International bureau of weights
and measures at severs near Paris in France.
3) 1
meter is equal to 1650763.73 times the
wavelength of orange red light
Emitted by Kr86 source, kept at
triple point of Nitrogen.
Many units to measure length
1 Km = 10^{3} m
1 deci meter = 10^{1} m
1 centi meter = 10^{2} m
1 milli meter = 10^{3} m
1 micro meter = 10^{6} m
1 angstrom = 10^{10} m
1 fermi = 10^{15} m
9 . Define 1 Kg
1 Kg is equal to the mass of Platinum – Iridium cylinder of diameter equal to
its height, kept at international bureau of weights and measures.

Definition of 1 Kg 
Many units to measure mass
1 Kg = 1000 gm
1 Ton = 1000 Kg
1 Quintal = 100 Kg
1 gm = 1000 mg
10. Define 1 second
(a) 1
second is the duration of 9,192,631,770 periods of the radiation corresponding
to the transition between the two hyperfine levels of Cesium – 133 atom in ground state.
(b) 1
second is defined as 1 / (86,400) th
part of a mean solar day.
Mean solar day is the average of
all solar days in a single year.
11. Define 1 Ampere
1 Ampere is that constant current
which if maintained in two straight parallel conductors of infinite length , of
negligible cross section and placed 1 m apart in vacuum , would produce between
these conductors a force equal to 2 x 10^{
7} N
12. Define 1 Radian
1 Radian is the plane angle between
two radii of a circle which cutts off on the circumference an arc equal to the
radius of the circle.

1 radian 
360 degree = 2
∏ radian
There for, 1 radian
= 180 / ∏ degree
13. Define 1 Steradian
1 Steradian is the solid angle
which with its vertex at the center of a sphere cuts off an area on the
surface of the sphere equal to that of a square having sides of length equal to
the radius of the sphere.
Order of Magnitude
To estimate how big or small a
given physical quantity , we can use the idea of
Order of magnitude.
The given magnitude is expressed
as a x 10 ^{b}
Where ‘a’ is a number between 1 and 10
Number ‘b’ can be +ve or –ve
If
a > 5 , then order of
magnitude = b+1
If
a < =5, then order of magnitude = b
For example :
Diameter of earth 1.28 x 10^{7} m
Order of magnitude = 7 and it is of the Order of 10^{7}
Distance between two points is measured as 7.25 x 10^{10} m
Its order of magnitude is 10^{9} and It is of the Order of 10^{9}