Engineering Mechanics
Efficiency of a screw jack is given by (where α = Helix angle, and φ = Angle of friction.)

None of these
tan(α + φ)/tanα
tan(α - φ)/tanα
tanα/tan (α + φ)

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Engineering Mechanics
A semicircular disc rests on a horizontal surface with its top flat surface horizontal and circular portion touching down. The coefficient of friction between semi circular disc and horizontal surface is µ. 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
None of these

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Engineering Mechanics
The static friction

All of these
Is independent of the area of contact, between the two surfaces
Always acts in a direction, opposite to that in which the body tends to move
Bears a constant ratio to the normal reaction between the two surfaces

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