Theory of Machine
Open pairs are those which have

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

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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

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

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