What Is Equilibrium

Equilibrium is a physical state and it has different definition in different disciplines of physics. Let us start from mechanical physics.

Mechanical Equilibrium 
According to mechanics, when the net force acting upon a particle is zero, then the body can be said as in equilibrium. Mechanical equilibrium can be of two types; first one is static equilibrium and the second one is dynamic equilibrium.

Static Equilibrium
A body is said to be in static equilibrium, when the body is in rest and the net force acting upon the body is zero.    The best example of static equilibrium is a car stands still on the road. When a car is standing still on the road, it can be said that the car is in equilibrium. However, there are two forces are acting upon it. First one, the weight of the car, which is directed towards the center of the earth and then the second one, which is normal force exerted by the road on the car. The magnitude of the normal force is equal to the magnitude of the car and the direction of the normal force is against the weight. So, these two forces cancel out each other and the net force on the car is zero.

Dynamic Equilibrium
When a body is in motion, but the net force acting upon the body is zero, then the body is said to be in dynamic equilibrium. The example of dynamic equilibrium is a car moving with a constant velocity on a frictionless track. When a car is moving with a constant velocity along a friction less track, then the car is said to be in dynamic equilibrium as only two forces are acting on the car, weight of the car and the normal force and both nullifies each other.

Further the mechanical equilibrium can be divided into two categories; stable equilibrium and unstable equilibrium.

Stable Equilibrium
If one applies a little force on a body, which is in equilibrium and after some time, the body again comes back to in the equilibrium position, then the body is said to be in stable equilibrium.

Unstable Equilibrium
When a slight force can disturbs the equilibrium of a body permanently, then the equilibrium of the body would be called as unstable equilibrium.   Thermal Equilibrium – When the temperature of a body doesn’t change by the time, then the body is called as in thermal equilibrium.

The two above mentioned equilibrium states are most renowned and frequently discussed equilibriums. Apart from these two, there are many other kind of equilibriums exist in physics and other academic disciplines and there are dozens of sub categories of those different equilibriums. However, in most of the cases, the equilibrium indicates a stable state.