Strength of Materials
A number of forces acting at a point will be in equilibrium if

Two resolved parts in two directions at right angles are equal
Sum of resolved parts in any two perpendicular directions are both zero
Their total sum is zero
All of them are inclined equally

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

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