EQUILIBRIUM

Definition 

When the resultant of all the forces acting on a body comes to zero, the body is said to be in the state of equilibrium. 
                                                                                             OR 
In other words the body is said to be in the state of equilibrium if left-ward forces are equal to the rightward forces and upward forces are equal to the downward forces. 
Example 
• A book lying on the table.
 • A train moving in a straight line with a constant velocity. 
• A body hanging at rest from the ceiling by a vertical string. 
• A Paratrooper moving down with uniform velocity after opening his parachute. 

TYPES OF EQUILIBRIUM  

There are two types of equilibrium. 
1. Static equilibrium 
2. Dynamic equilibrium 

STATIC EQUILIBRIUM 

Definition
If some forces are acting on a body vertically or horizontally and the body maintains its state of rest, Then it is called static equilibrium. 
Example 
• A book lying on the table. 
• A body hanging at rest from the ceiling by a vertical string. 

DYNAMIC EQUILIBRIUM 

Definition 
If some forces are acting on a body vertically or horizontally and the body maintains its state of motion, then it is called dynamic equilibrium. 
Example 
• Train moving with uniform velocity.
• Paratrooper falling down with uniform velocity. 

CONDITIONS OF EQUILIBRIUM  

FIRST CONDITION OF EQUILIBRIUM 

 Statement 
The body is said to be in the state of equilibrium if the resultant of all forces acting on a body is equal to zero i.e.

In other words the algebraic sum of all the forces acting along X-axis should be zero i.e.

And the algebraic sum of all the forces acting along Y-axis should be zero i.e. 

When first condition of equilibrium is satisfied. the body is in translational equilibrium and there is no linear acceleration. 

SECOND CONDITION  OF EQUILIBRIUM 

Statement The body is said to be in the state of equilibrium if the sum of all the torques or moments acting on the body must be zero i.e.