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

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

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Engineering Mechanics
The static friction

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

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