Engineering Mechanics
Angle of friction is the

Angle between normal reaction and the resultant of normal reaction and the limiting friction
Ratio of limiting friction and normal reaction
The ratio of minimum friction force to the friction force acting when the body is just about to move
The ratio of minimum friction force to friction force acting when the body is in motion

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Engineering Mechanics
Varingon's theorem of moments states that if a number of coplanar forces acting on a particle are in equilibrium, then

The algebraic sum of their moments about any point in their plane is zero
The algebraic sum of their moments about any point is equal to the moment of their resultant force about the same point
Their algebraic sum is zero
Their lines of action are at equal distances

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