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

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Theory of Machine
Kinematic pairs are those which have

Surface contact between the two elements when in motion
Elements of pairs not held together mechanically
Point or line contact between the two elements when in motion
Two elements that permit relative motion

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Theory of Machine
The contact ratio is given by

(Length of the path of approach)/(Circular pitch)
(Length of the arc of approach)/cosφ
(Length of path of recess)/(Circular pitch)
(Length of the arc of contact)/(Circular pitch)

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