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

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Applied Mechanics and Graphic Statics
The force acting on a point on the surface of a rigid body may be considered to act

At the centre of gravity of the body
At any point on the surface normal to the line of action of the force
On the periphery of the body
On any point on the line of action of the force

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