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

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

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Theory of Machine
Higher pairs are those which have

Two elements that permit relative motion
Point or line contact between the two elements when in motion
Surface contact between the two elements when in motion
Elements of pairs not held together mechanically

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