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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Applied Mechanics and Graphic Statics
The phenomenon of collision of two elastic bodies takes place because bodies

All listed here
Attempt to regain its original shape due to their elasticities
Tend to compress each other till they are compressed maximum possible
Immediately after collision come momentarily to rest

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