This document provides information about equilibrium and related concepts in physics. It defines equilibrium as a state where opposing forces neutralize each other such that there is no net force acting on an object. Key points discussed include: - The definition of equilibrium is derived from Latin words meaning "equal balance". - Equilibrium can be static, with zero net force and no movement, or dynamic, with zero net force but constant motion. - Newton's first law relates to equilibrium, stating that an object at rest stays at rest unless a net force acts on it. - For an object to be in equilibrium, the net force and net torque on it must equal zero. - Free body diagrams are used to visualize all

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Name : MD. Moinul Islam.
Student ID : 10.01.03.076.
Course No : CE-416.
Course Title: Prestress Concrete Design
Sessional.

3. The Word Equilibrium Derived From
• The word equilibrium is derived from the
Latin word æquilībrium, from æquus
(“equal”) + lībra (“balance”). which
means equal balance.
• In physics it means a state of even
balance in which opposing forces or
tendencies neutralize each other.

4. Equilibrium
• Is the condition of
force where it is
acted but simply
cancelled out. These
forces may be even
large enough to
cause permanent
deformation.

5. Basic Idea of Equilibrium

6. Terms Related To Equilibrium

7. CENTER OF GRAVITY
• The center of gravity of a body is the point where its
entire weight maybe assumed concentrated.
Practical example of equilibrium and center of gravity: A tight rope walker in a circus
carries a weighted pole or an umbrella.

8. Force System
Concurrent & Non-Concurrent
Concurrent system occur when the lines of actions of the forces
acting on a body intersect at a common point.
Non-concurrent system occurs when the forces are acting at
different points.

9. Types
There are two basic conditions of equilibrium.
Translational equilibrium.
Rotational equilibrium.
• The term "translational equilibrium" describes an
object that experiences no linear acceleration. (First
condition of equilibrium )
• An object experiencing no rotational acceleration (a
component of torque) is said to be in rotational
equilibrium. (Second condition of equilibrium )
• Typically, an object at rest in a stable situation
experiences both linear and rotational equilibrium.

10. Types
There are two kinds of mechanical equilibrium:
 static equilibrium and
 dynamic equilibrium.
• Any object which is in static equilibrium has zero
net force acting on it and is at rest.
• Any object which is in dynamic equilibrium has
zero net force acting on it and is moving at a
constant velocity.

11. Newton’s First Law of Physics:
• A body at rest will stay at rest and a body in
motion will stay in motion unless acted upon by
an unbalanced force.
• Therefore, sum of all forces must be zero.
F=0
Resultant of all forces
acting on a particle is
zero.
Equilibrium

12. Equilibrium Equation from Newton’s
Law
• If an object is in equilibrium, then the
resultant force acting on an object equals
zero. This is expressed as follows:
FR
F
0
(vectorequation)
Some problems can be analyzed using only 2D, while others require 3D.

13. Necessary Condition for Equilibrium
• The necessary conditions for equilibrium are:
(i) the vector sum of all external forces is zero.
(ii) the sum of the moments of all external
forces about any line is zero.

14. EQUATIONS OF EQUILIBRIUM
• As stated earlier, when a body is in equilibrium,
the net force and the net moment equal zero, i.e.
F = 0 and
M = 0
These two vector equations can be written as six scalar equations
of equilibrium. These are
FX = 0
FY = 0
FZ = 0
MX = 0
MY = 0
MZ = 0
6 equations for
3D equilibrium
Note: The moment equations can be determined about any
point. Usually, choosing the point where the maximum number
of unknown forces are present simplifies the solution.

15. Categories of equilibrium according to Force system

16. Three-Dimensional Reaction at Supports
& Connections

20. Free Body Diagram
(FBD)

21. THE WHAT, WHY AND HOW OF A FREE BODY
DIAGRAM (FBD)
Free Body Diagrams are one of the most important things for
you to know how to draw and use.
What ? - It is a drawing that shows all external forces acting on the particle.
Why ? - It is key to being able to write the equations of equilibrium—
which are used to solve for the unknowns (usually forces or angles).

22. Steps of Drawing a FBD
1. Imagine the particle to be isolated or cut free
from its surroundings.
2. Show all the forces that act on the particle.
Active forces: They want to move the particle.
Reactive forces: They tend to resist the
motion.
3. Identify each force and show all known
magnitudes and directions.

23. Steps of Drawing a FBD
Real world example
Idealized model
FBD

24. THE PROCESS OF SOLVING RIGID BODY
EQUILIBRIUM PROBLEMS
1) Draw a free-body diagram (FBD) showing all the
external forces.
2) Apply the equations of equilibrium to solve for
any unknowns.
Note: If there are more unknowns than the number
of independent equations, then we have a
statically indeterminate situation. We cannot
solve these problems using just statics.

25. APPLICATION
Straps
Ball-and-socket joint
Crane

26. Application
Journal bearing
Pinned connection

27. The tie rod from point A is used to support the overhang at the
entrance of a building. It is pin connected to the wall at A and to
the center of the overhang B.
If A is moved to a lower position D, will the force in the rod
change or remain the same? By making such a change without
understanding if there is a change in forces, failure might occur.

28. Thank You