Applied Mechanics and Graphic Statics
Kinetic friction may be defined as

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

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Applied Mechanics and Graphic Statics
If a set of given forces are such that their free vectors build a closed polygon, then

The resultant force is zero but resultant couple may not be zero
The resultant force and resultant couple both may not be zero
The resultant force is zero but resultant couple is not zero
The resultant force and resultant couple are always zero

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