Applied Mechanics and Graphic Statics
The following is in unstable equilibrium

A satellite encircling the earth
A uniform solid cone resting on its base on a horizontal plane
A uniform solid cone resting on a generator on a smooth horizontal plane
A solid cube resting on one edge

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Applied Mechanics and Graphic Statics
The characteristic of a couple is:

Algebraic sum of moments of forces, constituting a couple, about any point, is same
Algebraic sum of forces, constituting a couple is zero
All listed here
A couple can be never the balanced by a single force

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