Applied Mechanics and Graphic Statics
On a ladder resting on a rough ground and leaning against a smooth vertical wall, the force of friction acts

Upwards at its upper end
Downwards at its upper end
Perpendicular to the wall at its upper end
Zero at its upper end

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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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