Depending on the behavior of an object after being removed from its initial position, we can classify its equilibrium into three different types: stable, unstable and indifferent.
The behavior of a rigid body (a set of particles whose position does not vary with time), in a situation where its original position is changed, determines different types of equilibrium.
1) Stable Equilibrium: If an object, after being moved away from its equilibrium position, returns to it, we say that there is stable equilibrium. Bodies in stable equilibrium, after being removed from their initial position, have their center of mass (or center of gravity) changed to higher positions. Thus, the torque of the weight force causes the object to return to its initial position. This is what happens with the jester toy or with a simple cone.
3) Indifferent Equilibrium: When an object has its position altered and, even so, maintains its equilibrium situation, we say that there is indifferent equilibrium. This occurs whenever the body’s point of suspension coincides with its center of gravity. This is the case of a pen “lying” on a table. Even if there is a change in its position, the equilibrium situation will be maintained.