Question

A horizontal plank has a rectangular block placed on it the plank starts oscillating vertically and simple harmonically with an amplitude

Answer 1

The plank starts oscillating vertically and simple harmonically with an amplitude of 40 cm.

What happens is that the block loses contact with the plank  when the latter is at rest.

Then the  period of the oscillation is zero when it is on the topmost position

The omega square = g omega  that is equals to  40 cm when is later  converted into m then we get the answer.