Theory of Machine
The rotating shafts tend to vibrate violently at whirling speeds because

Bearing centre line coincides with the axis
The shafts are rotating at very high speeds
The system is unbalanced
Resonance is caused due to the heavy mass of the rotor

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Theory of Machine
For dynamic balancing of a shaft

None of these
The net couple due to the dynamic forces acting on the shaft is equal to zero
Both (A) and (B)
The net dynamic force acting on the shaft is equal to zero

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