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In equilibrium position of the block, extension in spring is x0 (say)
\ kx0 = mg .-(i)Now, if we displace the block by x in the downward direction, the net force on the block towards mean position is
F = k(x + x0) – mg = kx (using (i)) Hence the net force is acting towards mean position and is also proportional to x. So, the particle will perform SHM and its time period would be
T=2pie root m/ k