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

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

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Theory of Machine
A body is said to be under forced vibrations, when

A body vibrates under the influence of external force
None of these
There is a reduction in amplitude after every cycle of vibration
No external force acts on a body, after giving it an initial displacement

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