Theory of Machine
In a screw jack, the effort required to lower the load W is given by

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

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Theory of Machine
In a mechanism, the fixed instantaneous centers are those centers which

Moves as the mechanism moves, but joints are of permanent nature
None of the listed here
Vary with the configuration of the mechanism
Remain in the same place for all configurations of the mechanism

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