Theory of Machine
Higher pairs are those which have

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

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Theory of Machine
When a mass of a critically damped single degree of freedom system is deflected from its equilibrium position and released, then it will

Return to equilibrium position without oscillation
Oscillate with increasing time period
Oscillate with constant amplitude
Oscillate with decreasing amplitude

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