Stable and unstable equilibrium

This model demonstration shows the difference between stable and unstable equilibrium.

          

The equilibrium of a ruler supported on two round pens (or roller supports) can be achieved easily as shown in Fig. 1-4a. However, supporting the ruler horizontally on a single round pen alone is very difficult, because the external force (self-weight) from the ruler and the reaction force from the round pen are difficult to align exactly. If the ruler achieves equilibrium on the single round pen, this type of equilibrium is unstable and is not maintained if a slight disturbance is applied to the ruler or to the round pen. The ruler will rotate around the point of contact with the pen until it finds another support (Fig. 1-4b).