EM Lecture 1,2

Lecture 1,2
Engineering Mechanics
CE - 101
Lt Col Syed Mohsin Ali
[email protected]
 Lt Col Syed Mohsin Ali
 1997 – 2000
BE Mechanical Engineering (EME College)
 Gold Medal in Final Year Project
 2004 – 2006 and 2009 – 2010
OIC Design Centre – Military Vehicles Research &
Development Establishment
 2011 – 2012
MSc in Art & Science of Warfare (Staff Course)
 2013 – 2015
MSc in Mechanized Mining Systems (South Africa)
 2015 – 2016
Instructor Class-B at MCE (EM, FCP, CACE)
Class Introduction
Course Outline
Course Outline
 Credit Hours
Total = 3
Theory = 2.5
Practical = 0.5
 Number of Contact hours
Total = 64
Theory = 40
Practical = 24
Course Learning Outcomes
• Understand the basic terms used throughout
the course, geometrical properties of plane
areas and basic concepts about virtual work
and dynamics.
• Resolution of forces, moments, couples, two
dimensional force system and frictional forces;
and finding reaction and member forces in
simple beams and plane trusses.
• Verify some of the concepts through practical
in the laboratory.
Course Plan
Do’s and Don'ts
Introduction to Engineering
-What is Mechanics?
Mechanics is the science which describes and predicts
the conditions of rest or motion of bodies under the
action of forces
-Categories of Mechanics?
- Rigid bodies  A Rigid Body does not deform under load!
- Statics
- Dynamics
- Deformable bodies
- Fluids
Branches of Engineering Mechanics
Introduction to Engineering
Mechanics is the foundation of most engineering
sciences and is an indispensable prerequisite to
their study
Introduction to Engineering
- Rigid Body Mechanics
• Statics: deals with equilibrium of bodies under action
of forces (bodies may be either at rest or move with a
constant velocity).
Designing structures (Mechanical & Civil).
Introduction to Engineering
- Rigid Body Mechanics
• Dynamics: deals with motion of bodies (accelerated
Analyze responses of buildings to earthquakes (Civil)
& determine trajectories of satellites (Aerospace).
Introduction to Engineering
• Engineering Applications
– Describes how mechanics is applied in various
fields of engineering
– Emphasis on 2 essential aspects of engineering:
• Design – to choose parameters values to satisfy
stated design criteria
• Safety – to evaluate the safety of devices and
choose parameter values to satisfy stated safety
Fundamental Concepts
• Space & Time
– Space:
• 3-dimensional space & locations/positions of
points in space
• Distance between 2 points in space = length of
the straight line joining them
• SI unit of length: meter (m)
• U.S. Customary unit: foot (ft)
Fundamental Concepts
• Space & Time
– Time:
• Measured by the intervals between the events
• SI unit & U.S. Customary unit of time: second (s)
Fundamental Concepts
• Position of a point in space relative to some
reference point changes with time:
– Rate of change of position = velocity
• SI unit: meters per second (m/s)
• U.S. Customary unit: feet per second (ft/s)
– Rate of change of velocity = acceleration
• SI unit: meters per second squared (m/s2)
• U.S. Customary unit: feet per second squared
Fundamental Concepts
• Mass:
Quantity of matter in a body  measure of inertia of
a body (its resistance to change in velocity)
Fundamental Concepts
• Force:
The ‘push’ or ‘pull’ exerted by one body on another
 Characterized by its magnitude, direction of its
action, and its point of application.
 Force is a Vector quantity.
Fundamental Concepts
• Force:
Force acting on a body is related to the mass of the
body and the variation of its velocity with time.
Force can also occur between bodies that are
physically separated (Examples: gravitational,
electrical, and magnetic forces)
Fundamental Concepts
• Remember:
- Mass is a property of matter that does not change
from one location to another
- Weight refers to the gravitational attraction of the
earth on a body or quantity of mass. Its magnitude
depends upon the elevation at which the mass is
- Weight of a body is the gravitational force acting on it
Units and Dimensions
Mechanics - Idealization
• Particle:
A body with mass but with dimensions that can be
Mechanics - Idealization
• Particle:
A body with mass but with dimensions that can be
Three forces act on the hook at A.
All forces met at point A.
→ Hook can be assumed as a Particle
Mechanics - Idealization
• Rigid Body:
- A combination of large number of particles in which all
particles remain at a fixed distance (practically) from one
another before and after applying a load
- Material properties of rigid body are not required to be
considered when analyzing the forces acting on the body
- In most cases, actual deformations occurring in
structures, machines, mechanisms, etc. are relatively
small, and rigid body assumption is suitable for analysis
Newton’s Three Laws of Motion
Newton’s 1st Law of Motion
A particle originally at rest, or moving in a straight
line with constant velocity will remain in this state
provided particle is not subjected to an
unbalanced force
Newton’s Three Laws of Motion
Newton’s 1st Law of Motion
First Law contains the principle of the equilibrium
of forces  main topic of concern in Statics
Newton’s Three Laws of Motion
Newton’s 2nd Law of Motion
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 force
Newton’s Three Laws of Motion
Newton’s 3rd Law of Motion
The mutual forces exerted by 2 particles on each
other are equal in magnitude & opposite in
direction. The particles remains in state of
equilibrium only if exerted forces on them are
Newtonian Gravitation
• Gravitational force between 2
particles of mass m1 & m2 that
are separated by a distance ‘r’
G = universal gravitational constant
G = 6.673 × 10–11 Nm2/kg2
Newtonian Gravitation
• Weight of an object of mass m due to the gravitational
attraction of the earth is approximated by:
mE = mass of earth,
r = distance from the center of earth to
the object
Newtonian Gravitation
• Weight of an object at sea level (r = RE):
W  mg
• The value of g varies from location to location on the
surface of the earth.
– g = 9.81 m/s2 (SI units)
– g= 32.2 ft/s²
Newtonian Gravitation
Systems of units
- Very important for Engineers
- SI (System international) is used in Europe
- FPS ( Foot pound System) is used In USA
- Be careful when switching from one system to
the other
• U.S. Customary Units:
– Base units:
• Length: feet (ft)
• Force: pounds (lb)
• Time: second (s)
– Derived Unit:
• Mass: slug (the mass of material accelerated at
1 ft/s2 by a force of 1 lb)
• Newton’s 2nd law:
1 lb  1 slug  1 ft/s 2
1 slug  1 lb  s 2 /ft
Systems of units
– Force & mass are defined by the 2nd law
1 slug = Amount of matter accelerated at 1 ft/s2 when acted upon by
a force of 1 lb (slug = lb· s2/ft).
1 Newton = Force required to give 1 kg of mass an acceleration 1
m/s2 (N= 1kg· m/s2)
Systems of units
1 Newton is the force required to give a mass
of 1 kg an acceleration of 1 m/ s².
Weight is a force.
The weight of 1 kg Mass is:
W = mg
W = (1 kg)(9.81 m/ s²)
W = 9.81 N
Conversion of Units
Values must be expressed in terms of one system of
units before they are substituted into the equation
Conversion of Units
To express 1 mi/h in terms of ft/s:
 5280 ft 
1 mi/h  
  1.47 ft/s
 3600 s 
Conversion of Units
(a) 60 miles/h to ft/sec
(b) 100 lb.ft/s2 to kg.m/s2
(c) 20 slug/ft to kg/m
Units - Prefixes
• E.g. 1 kg = 1000 g, 1 Mg = 106 g = 1000 kg
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