Applied Mechanics and Graphic Statics
A heavy ladder resting on a floor and against a vertical wall may not be in equilibrium, if

Floor is smooth and the wall is rough
Floor and wall both are smooth surfaces
Floor is rough and the wall is smooth
Floor and wall both are rough surfaces

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Applied Mechanics and Graphic Statics
The following is in unstable equilibrium

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

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