Applied Mechanics and Graphic Statics
If the given forces P₁, P₂, P₃ and P₄ are such that the force polygon does not close, then the system will

Both (A) and (C)
Be in equilibrium
Always reduce to a resultant force
Always reduce to a couple

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Applied Mechanics and Graphic Statics
The angle of friction is:

The force of friction when the body is in motion
The ratio of the friction and the normal reaction
The angle between the normal reaction and the resultant of normal reaction and limiting friction
The force of friction at which the body is just about to move

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