Theory of Machine
The example of lower pair is

Shaft revolving in a bearing
All of these
Automobile steering gear
Straight line motion mechanisms

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

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

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