Theory of Machine
The two elements of a pair are said to form a higher pair, when they

Have a line or point contact when in motion
Permit relative motion
Have a surface contact when in motion
Are kept in contact by the action of external forces, when in motion

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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 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 force is equal to zero

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