Applied Mechanics and Graphic Statics
The centre of gravity of a homogeneous body is the point at which the whole

Weight of the body is assumed to be concentrated
All listed here
Area of the surface of the body is assumed to be concentrated
Volume of the body is assumed to be concentrated

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

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