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

Tend to compress each other till they are compressed maximum possible
Immediately after collision come momentarily to rest
Attempt to regain its original shape due to their elasticities
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Applied Mechanics and Graphic Statics
The characteristic of a couple is:

Algebraic sum of moments of forces, constituting a couple, about any point, is same
Algebraic sum of forces, constituting a couple is zero
A couple can be never the balanced by a single force
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