States of equilibrium:

There are three states of equilibrium: stable equilibrium, unstable equilibrium, and neutral equilibrium.

 

1. Stable equilibrium:

A body is said to be in stable equilibrium if after a slight tilt it returns to its previous position.

Example:

Consider a book lying on the table. Tilt the book slightly about its one edge by lifting it from the opposite side as shown in the figure. It returns to its previous position when set free. Such a state of the body is called stable equilibrium.

Features of stable equilibrium:

When a body is in stable equilibrium, its center of gravity is at the lowest position. When it is tilted, its center of gravity rises. It returns to its stable state by lowering its center of gravity. A body remains in stable equilibrium as long as the center of gravity acts through the base of the body.

Explanation:

Consider a block shown in the figure. When the block is tilted, its center of gravity G rises. If the vertical line through G passes through its base in the tilted position as shown in figure (b), the block returns to its previous position. If the vertical line through G gets out of its base as shown in figure(c), the block does not return to its previous position.

 

 

2. Unstable equilibrium:

If a body does not return to its previous position when set free after the slightest tilt is said to be in unstable equilibrium.

Example:

Take a pencil and try to keep it in the vertical position on its tip as shown in the figure. Whenever you leave it, the pencil topples over about its tip and falls. This is called an unstable equilibrium. Thus, a body is unable to keep itself in a state of unstable equilibrium.

Features of unstable equilibrium:

The center of gravity of the body is at its highest position in a state of unstable equilibrium. As the body topples over about its base (tip), its center of gravity moves towards its lower position and does not return to its previous position.

 

DO YOU KNOW?

Vehicles are made heavy at the bottom. This lowers their center of gravity and helps to increase their stability.

 

3. Neutral equilibrium:

If a body remains in its new position when disturbed from its previous position, it is said to be in a state of neutral equilibrium.

Example:

Take a ball and place it on a horizontal surface as shown in the figure. Roll the ball over the surface and leave it after displacing the fit ffrom its previous position. It remains in its new position and does not return to its previous position. This is called a neutral equilibrium. There are various objects which have neutral equilibrium such as a ball, a sphere, a roller, a pencil lying horizontally, an egg lying horizontally on a flat surface, etc.

 

 

Features of neutral equilibrium:

In neutral equilibrium, all the new states in which a body is moved are the stable states and the body remains in its new state. In neutral equilibrium, the center of gravity of the body remains at the same height irrespective of its new position.