Engineering Mechanics-Module 1 (Introduction)

University of Engineering & Technology Peshawar
CE-117: Engineering Mechanics
MODULE 1:
Introduction to Engineering Mechanics
(some of Fundamental concepts)
Prof. Dr. Mohammad Javed and Dr. Muhammad Fahim
drfahimuet@gmail.com
1

Contents
1. Introduction to Mechanics
2. Definitions
3. Newton’s Three Laws of Motion
4. Classification of Force
5. Types of Structural Loads
6. Units of Measurement
7. Famous Structural Failures

Lecture’s Objectives
• To define and discuss various branches of Mechanics
• To discuss some of fundamental concepts used in Engineering
Mechanics
• To discuss system of units used in Engineering

• Mechanics is the branch of Science which deals with the effect of
forces on bodies.
• Figure 1: What is the effect of 2kg mass (Force ?)
on attached cables (Body) ?
• Figure 2: What is the effect of 800 N reaction
(Force ?) on Tendon (Body ?)
Figure 1
Figure 2 4

• Figure 3: What is the effect of Force ‘F’
on rod (Body) of length ‘L’?
Figure 3
5

Mechanics
Rigid
bodies
Fluids
Deformable
bodies
Engineering Mechanics
Statics Dynamics
Kinematics Kinetics
(Engineering
Mechanics)
(Solid Mechanics)
(Fluid Mechanics)
1. Introduction to Mechanics: Branches

RIGID BODY MECHANICS
Rigid body:
Anybody which doesn’t undergo deformation (change in
length or change in area or change in shape) under the action
of forces is said to be rigid body
Deformable body Rigid body
Rigid body (Engineering Mechanics ) Deformable body (Mechanics of Solid)

Consider the given figure. The calculation of the tension in the cable
which supports the boom of a mobile crane under load is essentially
unaffected by the small internal deformations in the structural
members of the boom.
8
For the purpose of determining the
external forces which act on the boom, we
may treat it as a rigid body.
Actually solid bodies are never rigid; they
deform under the action of applied forces.
In many cases this deformation is negligible
compared to the size of the body and the
body is assumed to be rigid
RIGID BODY MECHANICS

• Statics: It is that branch of Engineering Mechanics, which deals with the forces and
their effects, while acting upon the bodies at rest or or moving with constant
velocity (acceleration = 0 ?).
• Dynamics: It is that branch of Engineering Mechanics, which deals with the forces
and their effects, while acting upon the bodies in motion. The subject of Dynamics
may be further sub-divided into the following two branches : 1. Kinetics, and 2.
Kinematics.
• Kinematics: Kinematics is the branch of mechanics which deals with motion
parameters without considering the forces responsible for motion.
• Kinetics: Kinetics is the branch of mechanics which deals with motion
parameters as well as forces responsible for motion.
CLASSIFICATION OF RIGID BODY MECHANICS
S= vit +1/2 at2
Fi= ma

Applications of Statics
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• Deformable Body: Bodies in which appreciable
deformation is produced under application of
load.
• Deformable Body Mechanics
Deformable body mechanics (Mechanics of solids
course) deals with how forces are distributed inside
bodies, and with the deformations caused by these
internal force distributions. These internal force
produce "stresses" in the body, which could ultimately
result in the failure of the material itself.
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DEFORMABLE BODY MECHANICS

The Mechanics of fluids is the branch of mechanics that deals with
liquids or gases.
 Fluids are commonly used in engineering applications. They can be
classified as incompressible, or compressible.
 While all real fluids are compressible to some degree, most liquids
can be analyzed as incompressible in many engineering
applications.
 Applications of fluid mechanics abound, from hydraulics and
general flow in pipes to air flow in ducts to advanced applications in
turbines and aerospace.
 The study of the mechanics of fluids will be studied in courses
called Fluid Mechanics, Hydraulics and other relevant subjects.
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FLUID MECHANICS

1. Mechanics of solids I &II (2nd & 3rd semester)
2. Fluid Mechanics I &II (3rd & 4th semester)
3. Hydraulics Engineering (5th semester)
4. Irrigation Engg and Water Management (8th semester)
5. Structural Analysis I &II (4th & 5th semester)
6. Reinforced concrete design I & II (6th & 7th semester)
7. Steel Structures (8th semester)
8. Geotechnical Engineering I &II (4th & 5th semester)
9. Foundation Engineering (6th semester)
10. Introduction to Structural Dynamics and Earthquake Engineering
(8th semester)
CE courses requiring knowledge of Engineering Mechanics as pre-requisite

2. Definitions
Space: It is the geometric region occupied by bodies whose positions
are described by linear and angular measurements relative to a
coordinate system. For three-dimensional problems, three independent
coordinates are needed. For two-dimensional problems, only two
coordinates are required.
Time: It is conceived as a succession of events. Although the principles
of statics are time independent, this quantity plays an important role in the
study of dynamics.
Two dimensional rectangular Coordinate system Three dimensional Coordinate system
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2. Definitions
Mass : The quantity of the matter possessed by a body is called
mass.
 The mass of a body will not change unless the body is damaged and
part of it is physically separated.
 When a body is taken out in a space craft, the mass will not change
but its weight may change due to change in gravitational force. Even
the body may become weightless when gravitational force vanishes
but the mass remain the same.
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2. Definitions
Particle: A particle is an object whose mass is concentrated at a
point. For this reason, a particle is also called a point mass, and
it is said to have zero volume
16
Size of earth is insignificant compared to
the size of its orbit. Earth can be
modelled as a particle when studying its
orbital motion
Body: A body has mass and occupies a volume of space

Force may be defined as any action that tends to change the state of
rest or motion of a body to which it is applied.
• The action of a force is completely characterized
by
1. its magnitude,
2. direction of its action, and
3. Its point of application.
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2. Definitions

• Engineering mechanics is formulated on the basis of Newton’s three
laws of motion, the validity of which is based on experimental
observation. These laws apply to the motion of a particle as
measured from a nonaccelerating reference frame.
• First Law: Every body continues in its state of rest or of uniform
motion unless or until some external force acts on it.
• A football will remain at rest
unless acted upon by unbalanced force
18

• Second Law: A particle acted upon by an unbalanced force F
experiences an acceleration a that has the same direction as
the force and a magnitude that is directly proportional to the
the force.
• If F is applied to a particle of mass m , this law may be expressed
mathematically as F = ma
19

Third Law: The forces of action and reaction between bodies in
contact are equal in magnitude, opposite in direction and
collinear (same line of action).
20
Examples of Newton third law of motion

Force
Body Force
For the application of such
forces contact between the
surfaces is not essential. e.g.,
gravitational force, magnetic
force etc.
Surface Force
For the application of this type
of force contact between the
surfaces is necessary i.e.
friction force, Reaction force of
the roller and hinge support
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Force
Concentrated
Force
Distributed
Force
23

24
1. Concentrated force concentrated
forces exerted at point or location
2. Distributed force
A force applied along a length or
over an area. The distribution can be
uniform or non-uniform.
Concentrated forces, P1, P 2
What are other concentrated forces ?
Distributed forces

External and Internal Effects of a force
External force
For the bracket of Fig. 2/1 the effects of P
external
to the bracket are the reactive forces (not
shown) exerted on the bracket by the
foundation and bolts because of the action of
P.
 Forces external to a body can be either
applied forces or reactive forces.
25
Applied force
What are the other external effects ?

Internal force
The effects of P internal to the bracket are the
resulting internal forces and deformations
distributed throughout the material of the
bracket.
 The relation between internal forces and
internal deformations depends on the
material properties of the body and is
studied in strength of material (also known
as Mechanics of Solids or Mechanics of
materials)
26
External and Internal Effects of a force

Principle of Transmissibility
“A force may be applied at any point on its given line of action without
altering the resultant effects of the force external to the rigid body on
which it acts“
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What are the external effects
on the given body ?

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Type of Internal forces
Tension

Stress is a term used to express
the internal force or resistance,
offered by a particle to the
adjacent particle in a body under
the action of external loads.
• Stress is the internal resistance
per unit area.
29

30
Torsional
Normal

31
Classification of Force system

TYPES OF FORCES & FORCE SYSTEM
collinear forces concurrent forces
Parallel forces Non-concurrent & non-parallel forces
Classification of Coplanar Force system

Load is a term frequently used in engineering to mean the
force exerted on a surface or body.
33
5. Types of Structural loads

Dead Loads: Vertical loads that are fixed in position and are
produced by the weight of the elements of the structure or
the whole structure with all its permanent components.
Examples are: own weight of structural member and super
imposed loads (e.g. walls and flooring cover)
34

Live Loads: consist mainly of occupancy
loads (e.g. people and furniture) in
buildings and traffic loads on bridges.
They may be either fully or partially in
place or not present at all, and may also
change in location.
35

Wind Loads: are the positive or
negative pressures exerted on a
building when it obstructs the flow
of moving air.
Wind loads generally act perpendicular
the surface of the structure.
Value of load varies depending on the
geographic location of the building
and its height.
36

Seismic Loads: are the inertial forces that act on the structure
due to earthquake-induced ground motions.
37
Ground acceleration, a
Inertial force
=ma

Snow Loads: the amount of snow load on a roof structure is
dependent on a variety of factors:
• Roof geometry,
• Size of the structure,
• Insulation of the structure,
• Wind frequency,
• Snow duration,
• Geographical location of the structure.
38

Lateral Soil and Hydro-static Loads:
39
5. Type of Structural loads

Thermal and Settlement Loads:
40
5. Type of Structural loads
Thermal Loads Settlement Loads

• In mechanics we use four fundamental quantities called
fundamental mechanical dimensions. These are length,
mass, force, and time.
• Although there are a number of different systems of units
major systems of units are
1. The International System of Units (SI units)
2. U.S. Customary Units
41

SI Units. The International System of units, abbreviated SI is a modern
version of the metric system which has received worldwide
recognition.
• SI system defines length in meters (m), time in seconds (s),and mass
in kilograms (kg).
• The unit of force, called a newton (N), is derived from F = ma. Thus, 1
newton is equal to a force required to give 1 kilogram of mass an
acceleration of 1 m/s2 (N = kg .m/s2) .
42

• U.S. Customary. In the U.S. Customary system of units (FPS) length is
measured in feet (ft), time in seconds (s), and force in pounds (lb).
• The unit of mass, called a slug , is derived from F = ma . Hence,
• 1 slug is equal to the amount of matter accelerated at 1 ft/s2 when
acted upon by a force of 1 lb (slug = lb.s2/ft) .
43

 In U.S. units the pound is also used on occasion as a unit of mass.
When distinction between the two units is necessary, the force unit is
frequently written as lbf and the mass unit as lbm
 Also, in the U.S. units (some time also called FPS system of units),
following relations are used
 1 ft = 12 in. (inches), 1 yard (yd)= 3 ft, 1 mile (mi)= 1760 yd= 5280 ft ,
 1 kilo-pound (kip)= 1000 lb ; 1 ton = 2000 lb ; 1 tonne = 1000 kg*=
2205 lb
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* In the MKS (meter, kilogram, second) gravitational system, which has been used for many years in
non-English-speaking countries, the kilogram, like the pound, has been used both as a unit of force
and as a unit of mass.

Abbreviation for inch.
Note that in previous slide the abbreviation for inch is “in.”,
which contains a period. This is unusual, but is done because
without the period, the abbreviation would also be the same
as a word in the English language, and this might lead to
confusion.
45

Common pitfall
Weight and mass are different.
It is unfortunately common for people, especially lay
people, to refer to weight using mass units. For
example, when a person says, “I weigh 70 kg.”, the
person really means “My mass is 70 kg”
46

Dimensions versus units.
Dimensions and units are different.
Dimensions are a measurable extent of some kind, while
Units are used to measure a dimension.
For example, length and time are both dimensions, and
meter and second, respectively, are units used to measure
these dimensions
47

Common prefixes used in the SI unit systems.
48

Conversion of Units
• Following table provides a set of direct conversion factors between
FPS and SI units for the basic quantities.
49

• Example 1
50
Conversion of Units

• Problem 2
51
Conversion of Units

Omitting units in equations.
The most serious mistake made when performing unit
conversions (as well as when writing equations in
general) is to omit units in equations.
Although writing units in equations takes a few
moments longer, doing so will help avoid the errors
that are sure to result if you do not make this a
practice
52
Common pitfall

53
1.1
1. 2
1.3
Ans:24.6 m/s
Ans: 101 kPa
Ans:
Conversion of Units: Practice Problems

54
1. 5
1.4
Ans:
Ans: 98.1 N, 4.9mN, 44.1 kN
Conversion of Units: Practice Problems

Among all of the goals confronting engineers when they
design structures and machines, the most crucial goal is
to develop designs that are as safe as possible.
Unfortunately, despite all human efforts to meet this goal,
sometimes we do not, and for reasons that are almost
always unexpected, failure occurs.
When failure occurs, we must learn from it so that our
mistakes and/or lack of foresight is not repeated in the
future
55

1. Tacoma Narrows bridge
2. Escambia Bay bridge.
56

3. Kansas City Hyatt Regency Hotel.
On July 17, 1981, two suspended walkways at the Kansas City Hyatt
Regency hotel collapsed during a dance party, killing 114 people
and seriously injuring many more.
57

4. Tropicana Casino parking garage.
On October 30, 2003, a 10-story parking
garage under construction at the Tropicana
Casino and Resort in Atlantic City, New Jersey,
collapsed, killing 4 workers and injuring 21
others
The design of the building itself was adequate,
but the design of structures needed for
fabrication was not. Note that concrete requires
time after pouring (28 days is common) to
reach
its full design strength.
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Engineering Mechanics-Module 1 (Introduction)

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