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

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

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Applied Mechanics and Graphic Statics
Kinetic friction may be defined as

Ratio of limiting friction and normal reaction
Friction force acting when the body is just about to move
Friction force acting when the body is in motion
Angle between normal reaction and resultant of normal reaction and limiting friction

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