Applied Mechanics and Graphic Statics
The necessary condition of equilibrium of a body is:

Algebraic sum of vertical components of all the forces must be zero
All (A), (B) and (C)
Algebraic sum of the moments of the forces about a point must be zero
Algebraic sum of horizontal components of all the forces must be zero

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Applied Mechanics and Graphic Statics
Kinetic friction may be defined as

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

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