Theory of Machine
The relation l = (2/3).(j + 2) apply only to kinematic chains in which lower pairs are used. This may be used to kinematic chains in which higher pairs are used, but each higher pair may be taken as equivalent to

One lower pair and two additional links
Two lower pairs and two additional links
Two lower pairs and one additional link
Any one of these

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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
None of these
There is a reduction in amplitude after every cycle of vibration

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