Applied Mechanics and Graphic Statics
The characteristic of a couple is:

A couple can be never the balanced by a single force
All listed here
Algebraic sum of forces, constituting a couple is zero
Algebraic sum of moments of forces, constituting a couple, about any point, is same

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

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