Strength of Materials
Two non-collinear parallel equal forces acting in opposite direction

constitute a moment
balance each other
constitute a couple
constitute a moment of couple

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

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
reaction of any smooth surface with which the body is in contact
All of the listed here

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