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

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

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Strength of Materials
Coefficient 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 friction force acting when the body is just about to move
the friction force acting when the body is in motion

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