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

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

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Theory of Machine
A system of masses rotating in different parallel planes is in dynamic balance if the

Resultant force is equal to zero
Resultant force and resultant couple are both equal to zero
Resultant force is numerically equal to the resultant couple, but neither of them need necessarily be zero
Resultant couple is equal to zero

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