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Physics · Class 9 · Chapter 6: Equilibrium

8 min

Key points

Equilibrium is the state of a body in which there is no change in its motion
First condition of equilibrium: The sum of all forces acting on a body must be zero (ΣF = 0)
Second condition of equilibrium: The sum of all torques acting on a body about any point must be zero (Στ = 0)
Stable equilibrium: When a body returns to its original position after being slightly displaced
Unstable equilibrium: When a body moves further away from its original position after being slightly displaced
Neutral equilibrium: When a body remains in equilibrium in its new position after being displaced
Centre of gravity is the point where the whole weight of the body appears to act
For a body to be in complete equilibrium, both translational and rotational equilibrium conditions must be satisfied

Definitions

Equilibrium: Equilibrium is the state of a body in which there is no change in its motion.
Stable Equilibrium: A body is said to be in stable equilibrium if it returns to its original position when slightly displaced.
Unstable Equilibrium: A body is said to be in unstable equilibrium if it moves further away from its original position when slightly displaced.
Neutral Equilibrium: A body is said to be in neutral equilibrium if it remains in equilibrium in its new position when displaced.
Centre of Gravity: Centre of gravity of a body is the point where the whole weight of the body appears to act.

Worked example

Define equilibrium. Explain the three types of equilibrium with examples. (6 marks)

Equilibrium is the state of a body in which it remains at rest or continues to move with uniform velocity when no net external force acts upon it. Stable equilibrium: When a body is displaced slightly from its equilibrium position, it returns to its original position, e.g., a ball at the bottom of a bowl. Unstable equilibrium: When a body is displaced slightly, it moves further away from its equilibrium position, e.g., a pencil balanced on its tip. Neutral equilibrium: When displaced, the body remains in equilibrium at its new position, e.g., a ball on a flat surface.

Common mistakes

Students confuse stable and unstable equilibrium definitions
Forgetting to write both conditions of equilibrium in long answers
Not drawing proper diagrams for types of equilibrium
Mixing up centre of gravity with centre of mass concepts
Not stating the mathematical expressions ΣF = 0 and Στ = 0 clearly

Quick recap

Equilibrium occurs when no net force acts on a body, keeping it at rest or in uniform motion. Three types exist: stable (returns when displaced), unstable (moves away when displaced), and neutral (stays in new position). Examples include ball in bowl (stable), pencil on tip (unstable), and ball on flat surface (neutral). Always use exact textbook definitions.

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