Applied Mechanics and Graphic Statics
The necessary condition of equilibrium of a body is:

All (A), (B) and (C)
Algebraic sum of horizontal components of all the forces must be zero
Algebraic sum of vertical components of all the forces must be zero
Algebraic sum of the moments of the forces about a point must be zero

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Applied Mechanics and Graphic Statics
The following is not a law of static friction:

The force of friction always acts in a direction opposite to that in which the body tends to move
The magnitude of the limiting friction bears a constant ratio to the normal reaction between two surfaces
The force of friction depends upon the roughness of the surface
The force of friction is dependent upon the area of contact

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