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

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Engineering Mechanics
The principle of transmissibility of forces states that, when a force acts upon a body, its effect is

Same at every point on its line of action
Maximum, if it acts at the center of gravity of the body
Minimum, if it acts at the center of gravity of the body
Different at different points on its line of action

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