Applied Mechanics and Graphic Statics
The characteristic of a couple is:

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

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Applied Mechanics and Graphic Statics
Periodic time of body moving with simple harmonic motion, is

Inversely proportional to the square of its angular velocity
Directly proportional to the square of its angular velocity
Inversely proportional to its angular velocity
Directly proportional to its angular velocity

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