Theory of Machine
Whirling speed of the shaft is the speed at which

Combination of transverse and longitudinal vibration occurs
Shaft tends to vibrate in longitudinal direction
Shaft tends to vibrate vigorously in transverse direction
Torsional vibrations occur

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

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