Condition for Equilibrium

An object is in equilibrium if it satisfies Newtons' first law:

"An object at rest or moving in a straight line with constant velocity will remain in this state unless it is subjected to an unbalanced force"

Quite simply put, an object is in equilibrium if the sum of forces acting on it equals zero:

This equation is called the equation of equilibrium.

Engineering Mechanics (Statics) - Equilibrium of a Particle

Before we discuss "equilibrium" lets go back a little and see what we understand about the whole subject of Mechanics.

Mechanics is a subject that is concerned with what happens to the motion of an object when forces are acting on it.  Mechanics can be divided into three branches¹:

1. Rigid-body mechanics (under the assumption that forces acting on the object cause no deformation upon the object)
2. Deformable-body mechanics
3. Fluid mechanics

At this point in time we are concerned with rigid-body mechanics which can be further divided into two areas:

1. Statics
2. Dynamics

When we talk of equilibrium we are in the Statics area of rigid-body mechanics.

An object is said to be in equilibrium when it is static or moving at constant velocity (not accelerating)

Dynamics on the other hand deals with object that are accelerating.

Download Engineering Mechanics Lecture Notes

Please click the following for my blog notes on Engineering Mechanics.

Scalars and Vectors

Multiplication and Division of a Vector by a Scalar

Two Dimentional Vector Addision - Parallelogram law

Vector Subtraction and Resolution of a Vector

Coplanar Forces and their Rectangular Components

Addition of Coplanar Forces - Using Scalar Notation

Addition of Coplanar Forces - Using Cartesian Vector Notation

Equilibrium of a Particle

Condition for Equilibrium

Free Body Diagram (FBD)

Below is a list of engineering mechanics tutorials.  Click on the individual topic to download the notes and don’t forget to print a copy for the lectures.

Introduction

Force Vector Tutorial

Equilibrium of a Particle

Force System Resultant

As you probably notice the list is not complete, they will be in due time.  The lecture notes are based on your recommended textbooks.  You are encouraged to refer to them for a wider understanding of the subject.