Engineering Mechanics
The maximum efficiency of a screw jack is

(1 - tanφ)/(1 + tanφ)
(1 + tanφ)/(1 - tanφ)
(1 + sinφ)/(1 - sinφ)
(1 - sinφ)/(1 + sinφ)

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Engineering Mechanics
The static friction

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
Is independent of the area of contact, between the two surfaces

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