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Related to unstable equilibrium: neutral equilibrium
unstable equilibrium[¦ən′stā·bəl ē·kwə′lib·rē·əm]
An equilibrium state of a system in which any departure of the system from equilibrium gives rise to forces or tendencies moving the system further away from equilibrium; for example, mechanical equilibrium in which the potential energy is a maximum, as a sphere sitting on top of a hill.
The condition of a structure in a state of equilibrium; when a slight disturbance is applied to the structure and then removed, the structure does not return to its original equilibrium position. Compare with stable equilibrium.