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 at a point or an axis, fixed in space, around which a body is con-strained to turn
reaction of a rough surface of a body which rolls on it without slipping

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Strength of Materials
A semi-circular disc rests on a horizontal surface with its top flat surface horizontal and circular portion touching down. The coefficient of friction between semi-cricular disc and horizontal surface is i. This disc is to be pulled by a horizontal force applied at one edge and it always remains horizontal. When the disc is about to start moving, its top horizontal force will

remain horizontal
unpredictable
slant up towards direction of pull
slant down towards direction of pull

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