Applied Mechanics and Graphic Statics
Force polygon method is applicable for

Non-concurrent coplanar force system
Concurrent coplanar force system
Any coplanar force system
A system of parallel forces only

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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 and resultant couple are always zero
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 is zero but resultant couple is not zero

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