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

None of these
A body vibrates under the influence of external force
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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Theory of Machine
A system of masses rotating in different parallel planes is in dynamic balance if the

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

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