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

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

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Engineering Mechanics
Angle of friction is the

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

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