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

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

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Engineering Mechanics
One joule means that

Work is done by a force of 1 dyne when it displaces a body through 1 cm
Work is done by a force of 1 N when it displaces a body through 1 m
Work is done by a force of 1 kg when it displaces a body through 1 m
Work is done by a force of 1 g when it displaces a body through 1 cm

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