Strength of Materials
According to principle of moments

if a system of coplanar forces is in equilibrium, then their algebraic sum is zero
the algebraic sum of the moments of any two forces about any point is equal to moment of theiwesultant about the same point
if a system of coplanar forces is in equilibrium, then the algebraic sum of their moments about any point in their plane is zero
positive and negative couples can be balanced

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Strength of Materials
In the equation of virtual work, following force is neglected

reaction of any smooth surface with which the body is in contact
All of the listed here
reaction of a rough surface of a body which rolls on it without slipping
reaction at a point or an axis, fixed in space, around which a body is con-strained to turn

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