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

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Applied Mechanics and Graphic Statics
Parallelogram Law of Forces states, "if two forces acting simultaneously on a particle be represented in magnitude and direction by two adjacent sides of a parallelogram, their resultant may be represented in magnitude and direction by

Its shorter side
The diagonal of the parallelogram which passes through the point of intersection of the forces
Its longer side
The diagonal of the parallelogram which does not pass through the point of intersection of the forces

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