Strength of Materials
Coefficient of friction is the

the friction force acting when the body is in motion
ratio of limiting friction and normal reaction
the friction force acting when the body is just about to move
angle between normal reaction and the resultant of normal reaction and the limiting friction

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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
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 a rough surface of a body which rolls on it without slipping

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