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 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 and resultant couple both may not be zero

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Applied Mechanics and Graphic Statics
Newton's law of Collision of elastic bodies states that when two moving bodies collide each other, their velocity of separation

Is directly proportional to their velocity of approach
Is equal to the sum of their velocities of approach
Bears a constant ratio to their velocity of approach
Is inversely proportional to their velocity of approach

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