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

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

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Applied Mechanics and Graphic Statics
The following is not a law of static friction:

The force of friction depends upon the roughness of the surface
The magnitude of the limiting friction bears a constant ratio to the normal reaction between two surfaces
The force of friction is dependent upon the area of contact
The force of friction always acts in a direction opposite to that in which the body tends to move

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