The purpose of the session
• Discuss basic and derived units and
• Estimate the physical quantities
• Explain Newton’s laws of motion
• Explain Scalar and Vector
The International System of Units (SI)
The International System of Units (SI) defines seven units
of measure as a basic set from which all other SI units
are derived. The SI base units and their physical
• metre (m) for length
• kilogram(kg) for mass
• second(s) for time
• ampere (A)for electric current
• Kelvin(K) for temperature
• Candela(cd) for luminous intensity
• Mole(mol) for the amount of substance
Symbols are written in lowercase except when named after person.
Unit of length m
The metre is the length of the path travelled by
light in vacuum during a time interval of 1/299
792 458 of a second.
1 ⁄ 10,000,000 of the distance from the Earth's
equator to the North Pole measured on the
circumference through Paris
Dimension symbol; L
Unit of mass kg
The kilogram is the unit of mass; it is equal to
the mass of the international prototype of the
The mass of one litre of water. A litre is one
thousandth of a cubic metre.
Dimension symbol; M
Unit of Time s
The second is the duration of 9 192 631 770 periods
of the radiation corresponding to the transition
between the two hyperfine levels of the ground
state of the cesium 133 atom.
The day is divided in 24 hours, each hour divided in
60 minutes, each minute divided in 60 seconds.
A second is 1 ⁄ (24 × 60 × 60) of the day.
Dimension symbol T
Unit of electric current A
The ampere is that constant current which, if maintained
in two straight parallel conductors of infinite length, of
negligible circular cross-section, and placed 1 meter apart
in vacuum, would produce between these conductors a
force equal to 2 x 10-7 newton per meter of length.
The original "International Ampere" was defined
electrochemically as the current required to deposit 1.118
milligrams of silver per second from a solution of silver nitrate.
Compared to the SI ampere, the difference is 0.015%.
Dimension symbol; l
Unit of thermodynamic temperature
The Kelvin, unit of thermodynamic temperature,
is the fraction 1/273.16 of the thermodynamic
temperature of the triple point of water.
The Celsius scale: the Kelvin scale uses the
degree Celsius for its unit increment, but is a
thermodynamic scale (0 K is absolute zero).
Dimension symbol; θ
Unit of amount of substance
The mole is the amount of substance of a system
which contains as many elementary entities as
there are atoms in 0.012 kilogram of carbon 12; its
symbol is "mol.“
When the mole is used, the elementary entities must be
specified and may be atoms, molecules, ions, electrons,
other particles, or specified groups of such particles.
Atomic weight or molecular weight divided by the molar mass
constant, 1 g/mol.
Dimension symbol; n
Unit of luminous intensity
The candela is the luminous intensity, in a given
direction, of a source that emits monochromatic
radiation of frequency 540 x 1012 hertz and that
has a radiant intensity in that direction of 1/683
watt per steradian.
The candlepower, which is based on the light
emitted from a burning candle of standard
Dimension symbol; J
The International System of Units (SI) specifies a
set of seven base units from which all other
SI units of measurement are derived. Each of
these other units (SI derived units) is
either dimensionless or can be expressed as a
product of (positive or negative, but usually
integral) powers of one or more of the base
Name Symbol Quantity SI equivalency
Hertz Hz frequency s−1
Radian rad, angle dimensionless
Newton N force, weight kg⋅m⋅s−2
Pascal Pa pressure, stress kg⋅m−1⋅s−2
Joule J energy, work, heat kg⋅m2⋅s−2
watt W power, radiant flux kg⋅m2⋅s−3
coulomb C electric charge s⋅A
volt V voltage, electrical potential difference,
electromotive force kg⋅m2⋅s−3⋅A−1
farad F electrical capacitance kg−1⋅m−2⋅s4⋅A2
ohm Ω electrical resistance, impedance, reactance kg⋅m2⋅s−3⋅A−2
A SI prefix is a name that is added to the name of a basic
unit and which indicates whether that unit is a multiple
(or a fraction) of that unit.
For example, the prefix "kilo“
added to "meter" gives "kilometer", which is a unit 1 000
times LARGER than the base unit "meter". Similarly, the
prefix "milli" added to "gram" gives "milligram", which is
a unit 1 000 times SMALLER than the base unit "gram".
The table shown next slide lists the names of approved SI
A physical quantity (or "physical magnitude") is
a physical property of a phenomenon, body, or
substance, that can be quantified by measurement.
A physical quantity can be expressed as the
combination of a number – usually a real number –
and a unit or combination of units; for
example, 1.6749275×10−27 kg (the mass of
the neutron), or299792458 metres per
second (the speed of light).
Newton's laws of motion
Newton's laws of motion are three physical
laws that, together, laid the foundation
for classical mechanics. They describe the
relationship between a body and
the forces acting upon it, and its motion in
response to those forces. They have been expressed
in several different ways, over nearly three
centuries,and can be summarised as follows.
When viewed in an inertial reference frame, an
object either remains at rest or continues to
move at a constant velocity, unless acted upon
by a force.
The first law can be stated mathematically as
An object that is at rest will stay at rest unless a
force acts upon it.
An object that is in motion will not change its
velocity unless a force acts upon it.
The vector sum of the forces F on an object is
equal to the mass m of that object multiplied by
the acceleration vector a of the object: F = ma.
The net force on an object is equal to the rate of
change (that is, the derivative) of its linear
momentum p in an inertial reference frame:
When one body exerts a force on a second body, the second body
simultaneously exerts a force equal in magnitude and opposite in
direction on the first body.
The third law states that all forces between two objects exist in equal
magnitude and opposite direction: if one object A exerts a force FA on a
second object B, then B simultaneously exerts a force FB on A, and the two
forces are equal and opposite: FA = −FB. The third law means that all forces
are interactions between different bodies, and thus that there is no such
Thing as a unidirectional force or a force that acts on only one body.
Properties of vectors
• Two vectors are equal if they have the same magnitude and
the same direction.
• Just like scalars which can have positive or negative values, vectors
can also be positive or negative. A negative vector is a vector which
points in the direction opposite to the reference positive direction.
For example, if in a particular situation, we define the upward
direction as the reference positive direction, then a
force F1−→=30 N downwards would be a negative vector and could
also be written as F1−→=−30 N. In this case, the negative (-) sign
indicates that the direction of F1−→ is opposite to that of the
reference positive direction.
• A negative vector is a vector that has the opposite direction to the
reference positive direction.