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

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Applied Mechanics and Graphic Statics
The following statement is one of the laws of Dynamic friction

The magnitude of the kinetic friction bears a constant ratio to the normal reaction between two surfaces. The ratio being slightly less than that in the case of limiting friction
The force of friction always acts in a direction opposite to that in which a body is moving
All of these
For moderate speeds the force of friction remains constant but decreases slightly with the increase of speed

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