School Of Excellence
Directorate of Education Govt. of Delhi, 9968 0 66788,
Chapter–2: Units and Measurements
Need for measurement: Units of measurement; systems of units; SI
units, fundamental and derived units. Length,mass and time
measurements; accuracy and precision of measuring
instruments;errors in measurement; significant figures. Dimensions of
physical quantities, dimensional analysis and its applications.
UNITS AND MEASUREMENTS
Need for the measurement.
Physics is an exact science which gives an accurate knowledge about
the nature and the natural phenomena.
PHYSICAL QUANTITIES:
All those quantities which can be measured directly or indirectly and
in terms of which the laws of physics can be expressed are called
physical quantity. in terms of the relationships among the quantities
involved.
FUNDAMENTAL AND DERIVED
Fundamental quantities.
The physical quantities which can be treated as independent of other
physical quantities and are not usually defined in terms of other
physical quantities are called fundamental quantities
Seven fundamental or base quantities.
These are mass, length, time, electric current, temperature, luminous
intensity and amount of substance
Derived quantities. The physical quantities whose defining operations
are based on other physical quantities are called derived quantities.
All physical quantities other than the seven base quantities are
Physics By: Dr.Hemendra Kumar, M.Sc.,M.Ed.,C.I.C.,Ph.D.(Physics), Lecturer Physics, P a g e | 1
Doubts: What’s app at 9968 0 66788
, School Of Excellence
Directorate of Education Govt. of Delhi, 9968 0 66788,
derived quantities. For example, velocity, acceleration, force,
momentum, etc.
Supplementary Quantity
Measurement.
The measurement of a physical quantity is the process of comparing
this quantity with a standard amount of the physical quantity of the
same kind, called its unit.
∴ Measure of a physical quantity
= Numerical value of the physical quantity × size of the unit
or Q = nu
Let length of a room = 5 m = 500 cm.
Q = n1 u1 = n2 u2 .
Physical unit.
The standard amount of a physical quantity chosen to measure the
physical quantity of the same kind is called a physical unit.
Characteristics of a physical unit:
1. It should be well-defined.
2. It should be of convenient size, i.e., neither too small nor too large
in comparison with the measurable physical quantity.
3. It should not change with time.
4. It should be easily reproducible.
5. It should be imperishable or indestructible
6. It should not be affected by the change in physical conditions such
as pressure, temperature, etc.
7. It should be internationally acceptable.
System of units.
A complete set of units which is used to measure all kinds of
fundamental and derived quantities is called a system of units.
Physics By: Dr.Hemendra Kumar, M.Sc.,M.Ed.,C.I.C.,Ph.D.(Physics), Lecturer Physics, P a g e | 2
Doubts: What’s app at 9968 0 66788
, School Of Excellence
Directorate of Education Govt. of Delhi, 9968 0 66788,
Some of the commonly used systems of units are as follows :
(i) cgs system. It was set up in France. It is based on
centimetre, gram and second as the fundamental units of
length, mass and time respectively.
(ii) fps system. It is a British system based on foot, pound and
second as the fundamental units of length, mass and time
respectively.
(iii) mks system. It is also a French system based on metre,
kilogram and second as the fundamental units of length,
mass and time respectively.
(iv) SI: The international system of units. SI is the abbreviation for
"Systeme Internationale d' Unites", which is French equivalent for
international system of units. It is a modernized and extended form of
the metric systems like cgs and mks systems. This system was
adopted by eleventh General Conference of Weights and Measures in
1960. It covers all branches of science and technology. It is based on
the following seven basic units and two supplementary units.
Table Basic SI quantities and units
s. Basic physical quantity Basic unit Symbol
No.
1. Length metre m
2. Mass kilogram kg
3. Time second s
4. Temperature kelvin K
Physics By: Dr.Hemendra Kumar, M.Sc.,M.Ed.,C.I.C.,Ph.D.(Physics), Lecturer Physics, P a g e | 3
Doubts: What’s app at 9968 0 66788
Directorate of Education Govt. of Delhi, 9968 0 66788,
Chapter–2: Units and Measurements
Need for measurement: Units of measurement; systems of units; SI
units, fundamental and derived units. Length,mass and time
measurements; accuracy and precision of measuring
instruments;errors in measurement; significant figures. Dimensions of
physical quantities, dimensional analysis and its applications.
UNITS AND MEASUREMENTS
Need for the measurement.
Physics is an exact science which gives an accurate knowledge about
the nature and the natural phenomena.
PHYSICAL QUANTITIES:
All those quantities which can be measured directly or indirectly and
in terms of which the laws of physics can be expressed are called
physical quantity. in terms of the relationships among the quantities
involved.
FUNDAMENTAL AND DERIVED
Fundamental quantities.
The physical quantities which can be treated as independent of other
physical quantities and are not usually defined in terms of other
physical quantities are called fundamental quantities
Seven fundamental or base quantities.
These are mass, length, time, electric current, temperature, luminous
intensity and amount of substance
Derived quantities. The physical quantities whose defining operations
are based on other physical quantities are called derived quantities.
All physical quantities other than the seven base quantities are
Physics By: Dr.Hemendra Kumar, M.Sc.,M.Ed.,C.I.C.,Ph.D.(Physics), Lecturer Physics, P a g e | 1
Doubts: What’s app at 9968 0 66788
, School Of Excellence
Directorate of Education Govt. of Delhi, 9968 0 66788,
derived quantities. For example, velocity, acceleration, force,
momentum, etc.
Supplementary Quantity
Measurement.
The measurement of a physical quantity is the process of comparing
this quantity with a standard amount of the physical quantity of the
same kind, called its unit.
∴ Measure of a physical quantity
= Numerical value of the physical quantity × size of the unit
or Q = nu
Let length of a room = 5 m = 500 cm.
Q = n1 u1 = n2 u2 .
Physical unit.
The standard amount of a physical quantity chosen to measure the
physical quantity of the same kind is called a physical unit.
Characteristics of a physical unit:
1. It should be well-defined.
2. It should be of convenient size, i.e., neither too small nor too large
in comparison with the measurable physical quantity.
3. It should not change with time.
4. It should be easily reproducible.
5. It should be imperishable or indestructible
6. It should not be affected by the change in physical conditions such
as pressure, temperature, etc.
7. It should be internationally acceptable.
System of units.
A complete set of units which is used to measure all kinds of
fundamental and derived quantities is called a system of units.
Physics By: Dr.Hemendra Kumar, M.Sc.,M.Ed.,C.I.C.,Ph.D.(Physics), Lecturer Physics, P a g e | 2
Doubts: What’s app at 9968 0 66788
, School Of Excellence
Directorate of Education Govt. of Delhi, 9968 0 66788,
Some of the commonly used systems of units are as follows :
(i) cgs system. It was set up in France. It is based on
centimetre, gram and second as the fundamental units of
length, mass and time respectively.
(ii) fps system. It is a British system based on foot, pound and
second as the fundamental units of length, mass and time
respectively.
(iii) mks system. It is also a French system based on metre,
kilogram and second as the fundamental units of length,
mass and time respectively.
(iv) SI: The international system of units. SI is the abbreviation for
"Systeme Internationale d' Unites", which is French equivalent for
international system of units. It is a modernized and extended form of
the metric systems like cgs and mks systems. This system was
adopted by eleventh General Conference of Weights and Measures in
1960. It covers all branches of science and technology. It is based on
the following seven basic units and two supplementary units.
Table Basic SI quantities and units
s. Basic physical quantity Basic unit Symbol
No.
1. Length metre m
2. Mass kilogram kg
3. Time second s
4. Temperature kelvin K
Physics By: Dr.Hemendra Kumar, M.Sc.,M.Ed.,C.I.C.,Ph.D.(Physics), Lecturer Physics, P a g e | 3
Doubts: What’s app at 9968 0 66788
