Engineering Mechanics
In a screw jack, the effort required to lift the load is given by (where W = Load lifted, α = Helix angle, and φ = Angle of friction.)

P = W tan (α + φ)
P = W tan (α - φ)
P = W tan (φ - α)
P = W cos (α + φ)

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Engineering Mechanics
Which of the following statement is incorrect?

A force acting in the opposite direction to the motion of the body is called force of friction
The ratio of the limiting friction to the normal reaction is called coefficient of friction
The velocity ratio of a machine is the ratio of load lifted to the effort applied
A machine whose efficiency is 100% is known as an ideal machine

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