# Vietnam University entrance Exam problem (Don't be fooled)

A light springs have stiffness coefficient $$K=20\text{ N/m}$$ hanging on to a fixed point, the lower of a string attaching small object $$A$$ weighing $$100\text{ g}$$; $$A$$ is connected to a small object $$B$$ weighing $$100\text{ g}$$ with a soft rope, piece, lightweight, non-stretch and long enough. From the equilibrium position of the system, pull down $$B$$ vertically to $$20 \text{ cm}$$ and then release with zero initial velocity. When object B began to reverse its movement, it suddenly slip from rope. Ignore any obstacles, take $$g = 10\text{ m/s}^2$$. The time between when the object $$B$$ were come out off the rope until fall to the initial position when $$B$$ was released is?

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