Eng. Ahmed Baasaay

2. Lecturer: Eng. Ahmed Baasaay

3. Mechanics is a branch of the physical sciences
that is concerned with state of rest or motion
of bodies that are subjected to the action of
forces.
What is Mechanics?

4. 9/12/2020Chapter 1 General Principles 4
Statics Dynamics
Rigid Bodies Deformable Bodies
Solid Mechanics Fluid Mechanics
Engineering Mechanics
1. Kinetics
2. Kinematics

5. In this course we will study rigid-body mechanics since
it is a basic of deformable bodies and the mechanics of
fluids.
Rigid body is essential for the design and analysis of
many types of structural members and mechanical
components.
Mechanics of rigid body is divided into two parts
which are Statics and Dynamics

6. Statics
 Deals with forces and its effects
when the body is at rest
Dynamics
 Deals with forces and its effects when
the body is in moving condition
TrussBridge IC Engine
Building Moving car

7. Fundamental Concepts
Particle: Body of negligible dimensions
Rigid body: Body with negligible deformations
Non-rigid body: Body which can deform
In Statics, bodies are
considered rigid unless
stated otherwise.
Before considering
whether the body can
be assumed rigid-body
or not,
you need to estimate
the relevant force first.

9. 9/12/2020Chapter 1 General Principles 9
Basic Quantities
Length
• meter
• foot
Time
• second
Mass
• kilogram
• slug
Force
• newton
• pound

10. Length
Needed to locate the position of a point in space
and describe the size of a physical system.
Time
Conceived as a succession of events. Concepts
of STATICS are time independent.
A property of matter by which we can compare the action of one
body to another. This property manifests itself as a gravitational
attraction between two bodies and provide a qualitative measure of
the resistance of matter to a change in velocity.
Mass

11. Force
Generally considered as a push of a pull exerted by
one body on another. Interaction occurs when there is
direct contact between the bodies.
Gravitational, electrical and magnetic forces do not
require direct contact.
Force is characterized by magnitude, direction and
point of application.

12. Idealizations
1. Particle - an object having mass but the size is neglected.
1. Rigid Body - a combination of a large number of particles
which remain in a fixed position relative to each other, both
before and after the application of a force.

15. NEWTON’S LAWS OF MOTION (1st Law)
The study of rigid body mechanics is
formulated on the basis of Newton’s laws
of motion.

v v
0F
First Law:
An object at rest tends to stay at rest and an object in
motion tends to stay in motion with the same speed and in
the same direction, unless acted upon by an unbalanced
force.

16. NEWTON’S LAWS OF MOTION (2nd Law)
Second Law:
The acceleration of a particle is proportional to the vector
sum of forces acting on it, and is in the direction of this
vector sum.
mF

a
r
amF



17. NEWTON’S LAWS OF MOTION
Third Law:
The mutual forces of action and reaction between
two particles are equal in magnitude, opposite in
direction,
and collinear.
F
r
F
r
F
r
F
r
Confusing?
Concept of FBD (Free Body Diagram)
Point: Isolate the body
Forces always occur in pairs – equal and
opposite action-reaction force pairs.

18. Newton’s Law of Gravitation
2
r
GMm
F 
- M & m are particle masses
- G is the universal constant of gravitation,
6.673 x 10-11 m3/kg-s2
- r is the distance between the particles.
For Gravity on earth (at sea level)
where
- m is the mass of the body in question
- g = GM/R2 = 9.81 m/s2 (32.2 ft/s2)
m

v r
W mg
M
m
r
F
M
W=mg