Equilibrium of a Particle in Physics with Key Concepts, Types, and Formula

Physics Laws of Motion 2025

Syed Aquib Ur Rahman
Updated on Jul 29, 2025 19:51 IST

By Syed Aquib Ur Rahman, Assistant Manager

Equilibrium of a particle in physics refers to an object’s current state. It’s a direct application of Newton’s First Law that states that an object will remain in its current state, be it rest or uniform motion, unless some external force disturbs it. Follow along with this concept of particle equilibrium, detailing conditions and types. All of these will help you later in solving problems in mechanics, and while visualising complex motion of bigger, real-world objects, covered in Class 11 Physics.   

Equilibrium of a particle

 

Table of content
  • Equilibrium of Particles Explanation
  • Conditions for Equilibrium of Particle
  • Types of Equilibrium of a Particle
  • Free Body Diagrams: How to Visualise Forces in Particle Equilibrium
  • Revise NCERT Physics Class 11 Chapters
  • Solve NCERT Solutions Physics in Class 11 Confidently
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Equilibrium of Particles Explanation

The NCERT book’s section 4.8 mentions:

Equilibrium of a particle in mechanics refers to the situation when the net external force on the particle is zero.

What are the implications when a particle is in equilibrium?

  • It satisfies Newton’s First Law of Motion. Being at equilibrium indicates there’s no net force that causes any acceleration on the object. 
  • There are two states of equilibrium, one at rest (static) and the other while moving (dynamic). 

Learning about the equilibrium of a particle helps engineers determine whether objects, including bridges, buildings, hanging signs, and moving vehicles, will stay put (static) or keep moving (dynamic) at a steady speed. 

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Conditions for Equilibrium of Particle

For a particle to be in mechanical equilibrium, the net external force from all directions acting on the particle must be zero

F = 0

This is the first condition of equilibrium of a particle. That means, since there is no acceleration, i.e., a=0, then when suing F=ma, based on the special case of the 2nd Law of Motion, we still get to learn that there is no net external force. 

Now, since we are also considering the forces that occur from all sides, that is, coordinates, these forces from all sides must be balanced

 

If we consider the forces in each coordinate direction x , y , both are balanced.

Mathematically, we can say that as

F x = 0 , F y = 0

In three dimensions, F z = 0 will also apply. 

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Types of Equilibrium of a Particle

Equilibrium is of two types, or you could say, there are two states, which ideally follow the First Law of Motion. They also follow the mechanical equilibrium condition that the net external force is zero. 

  • Static Equilibrium: This occurs for a particle at rest, where velocity is zero.  A classic example of static equilibrium is a book on a table that is at rest until you push it. 
  • Dynamic Equilibrium: The particle moves with constant velocity, with no acceleration. Consider the basic example of a car cruising at a constant velocity on a straight road.
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Free Body Diagrams: How to Visualise Forces in Particle Equilibrium

A free-body diagram (FBD) is essential for analysing equilibrium.

With it, you can show all common forces in mechanics acting on the particle in equilibrium. 

  • Gravitational force ( mg ).
  • Normal force ( N ).
  • Tension (T).
  • Frictional force ( f ).

For instance, have a look at this FBD. 

Basic example of FBD
What this FBD is saying is that this is a generalised free-body diagram for a particle in equilibrium. To analyse the forces, we represent the particle as a dot. We draw each force as a vector arrow originating from the dot. This example shows a downward gravitational force (mg), an upward normal force (N), a tension force (T) to the right, and a frictional force (f) to the left.

 

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Revise NCERT Physics Class 11 Chapters

Here are all the Physics Class 11 notes on all the chapters. Bookmark to revise them anytime!

Units and Measurements Class 11 Notes Mechanical Properties of Solids Class 11 Notes
Motion in a Straight Line Class 11 Notes Mechanical Properties of Fluids Class 11 Notes
NCERT Class 11 Notes for Motion in a Plane Thermal Properties of Matter Class 11 Notes
Laws of Motion Class 11 Notes Thermodynamics Class 11 Notes
Work, Energy, and Power Class 11 Notes Kinetic Theory of Gas Class 11 Notes
System of Particles and Rotational Motion Class 11 Notes Oscillations Class 11 Notes
Gravitation Class 11 Notes Waves Class 11 Notes

Click on these additional links for Class 11 Science revision notes.

NCERT Class 11 Notes for PCM
NCERT Class 11 Physics Notes

 

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Solve NCERT Solutions Physics in Class 11 Confidently

Q&A Icon
Commonly asked questions
Q:  

What is the basic concept of equilibrium?

A: 

Equilibrium in physics tells us about a situation where an object's motion or resting situation does not change. This can mean the object is either at rest (static) or moving at a steady speed (dynamic). This state occurs when there's no net external force or torque acting on the object. This thought follows Newton's First Law, which we know that an object will maintain its state of motion unless a force acts upon it. 

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