Applied Mechanics and Graphic Statics
The phenomenon of collision of two elastic bodies takes place because bodies

Attempt to regain its original shape due to their elasticities
Tend to compress each other till they are compressed maximum possible
Immediately after collision come momentarily to rest
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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 both may not be zero
The resultant force is zero but resultant couple is not 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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