Chapter (1)
Introduction
What is physics?
 Units
 Dimensional Analysis
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What is physics?
Physics is the most basic of all sciences.
 Underlying principles of all engineering fields.
 The goal of Physics is to explain on a fundamental
level how our environment behaves.
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Physicists try to model the world mathematically.
Physicists try to understand all the little pieces that
make a physical system work.
Classical physics
(Between 1600 and 1900)
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Classical mechanics: The study of the motion
of particles and fluids.
Thermodynamics: The study of temperature,
heat transfer, ……
Electromagnetism: Electricity, magnetism,
Electro- magnetic waves, and optics.
Modern physics
(After 1905)
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Special relativity: a theory of the behavior of
particles moving at high speed.
Quantum mechanics: A theory of
submicroscopic world of the atom.
General relativity: a theory that relates the
force of gravity to the geometrical properties of
space.
Some Major Fields of Physics
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Astrophysics
Atomic Physics
Biophysics
Chemical Physics
Geophysics
Nuclear Physics
Optics
Particle Physics
Solid State Physics
Units
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The value of physical quantity must be
expressed in terms of some standard or
units.
Units are necessary to compare
measurements and also to distinguish
between different quantities.
Different systems of units
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The international system of units (SI).
The British system.
The units of length is the foot.
 The units of time is the second.
 The units of Force is the pound.
 This system doesn’t use mass as a basic unit.
o
 The units of temperature is F.
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The international system of units
It uses seven base units:
 Length (m)
 Mass (kg)
 Time (s)
 Temperature (K)
 Electric current (A)
 Amount of substance (mol)
 Luminous intensity (cd)
Dimensional Analysis
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Since equations must be dimensionally correct,
dimensional analysis may be used to:
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Check the correctness of the form of the equation.
Derive a relation between the physical quantities.
Dimensional Analysis
physical
quantity
Units (SI)
Dimensions
Length
Mass
Time
Area
Volume
Force
Velocity
Acceleration
m
Kg
S
m2
m3
Kg m/S2
m/S
m/S2
L
M
T
L2
L3
M L / T2
L/T
L / T2
Check the correctness of an equation.
The speed of waves in shallow water depends only on the
acceleration of gravity g, with dimensions L/T 2, and on the
water depth h. Which of the following formulas for the wave
speed v could be correct?
1 2
v  gh
v  gh
2
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L L 2
 2L
T T
L
L

L
2
T
T
L L3
 2
T T
L L

T T
Terms do not match
Terms match,
this could be a valid formula.
Limitations
Dimensional analysis only checks the units.
Numeric factors have no units and can’t be tested.
gh
v
3
is valid.
v  gh  4
is not valid.
Example
The period p of a simple pendulum is the time for one complete
swing. How does p depends on the the mass m of the bob, the
length L of the string, and the acceleration due to gravity g?
P = K L a gb
(K is constant) ---------------- (1)
T = K [L]a [L/T2]b
Equate the power of each dimension, thus
L: 0 = a + b, then a = -b
T: 1 = -2b
then b = - 1/2,
Sub. In (1)
L
pk
g
a = 1/2
Example
The wavelength λ of a wave depends on the speed v of the wave and
its frequency f. decide which of the following equations is correct,
 = v/f or  = vf
Assume that = va fb
Solution
[L] = [L/T]a.[1/T]b
[L] = [L]a.[T]-a-b
Equate the power of each dimension, thus
L: 1 = a
T: 0 = -a-b
a = -b
 = v/f
QUIZ (1)
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What are the dimensions of the following?
[3]
[sin (wt)]
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Show that the following equation is Dimensionally correct.
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1 2
X  vot  at
2