The concept of equilibrium is a very important topic for NEET, JEE mains, JEE Advanced and of course for class 11 and 12. You are sure to get at least one question in any of the above exams where you would need to use this concept. So, learn this concept thoroughly to be able to solve those questions.

This concept of equilibrium is first introduced in chapter 5 of class 11 physics. In this chapter you will learn about 'Translational Equilibrium'. Later in chapter 7, you will learn about 'Rotational Equilibrium'.  Don't worry about the meanings of these terms now; they will become clear when you will learn chapter 7.

In the following discussions,  by the term 'equilibrium' we will mean 'Translational Equilibrium' only.

Definition of EQUILIBRIUM: The equilibrium of a particle in mechanics refers to the situation when the net external force  on the particle is zero.

We can understand from the above definition that:

(i) The vector sum of all the forces acting on the body must be zero.

(ii) As the net force on the body is zero, hence the acceleration of the body is zero.

(iii) So, if the body is at rest, it will continue to remain at rest, whereas, if the body was in motion, it will continue to move with constant velocity. (We can easily understand this from Newton's first law of motion).

From point (iii) above, we can divide equilibrium into two types:

  1. STATIC EQUILIBRIUM: When the body in equilibrium is at rest, it is called static equilibrium.
  2. DYNAMIC EQUILIBRIUM: When the body in equilibrium is in motion, it is called dynamic equilibrium.

Equilibrium is also divided another way into three types (or categories):

(a) Stable equilibrium

(b) Unstable equilibrium

(c) Neutral equilibrium

You must understand the above three types clearly to do well in entrance exams.

So, Let's now understand these three types of equilibrium with the following video lecture:

STABLE EQUILIBRIUM: Say we disturb a body in equilibrium by a little amount. If it comes back to its equilibrium position by itself, then it is called Stable Equilibrium.

UNSTABLE EQUILIBRIUM: Say we disturb a body in equilibrium by a little amount. If it moves further away from its equilibrium position, then it is called Unstable Equilibrium.

NEUTRAL EQUILIBRIUM: When a body stays in equilibrium at its new position after being disturbed a little from its equilibrium position, it is called neutral equilibrium.

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