Applied Mechanics and Graphic Statics
The following is in unstable equilibrium

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 satellite encircling the earth
A solid cube resting on one edge

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Applied Mechanics and Graphic Statics
The angle of friction is:

The force of friction when the body is in motion
The angle between the normal reaction and the resultant of normal reaction and limiting friction
The force of friction at which the body is just about to move
The ratio of the friction and the normal reaction

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