A coin is placed on a horizontal platform which undergoes vertical simple harmonic motion of angular frequency ω. The amplitude of oscillation is gradually increased. Thecoin will leave contact with the platform for the first time

The net effect of these two forces must be towards mean position.At the mean position, there is no net force and hence normal reaction equals mg. Above mean position, normal reaction is less than mg and below mean position, normal reaction is greater than mg.At the top extreme:N is minimum at the top extreme.Net force towards mean position = ma = mω2A⇒mg-Nt = mω2A   Nt = Nmin= mg-mω2AFor loosing contact with the platform, N= 0So, A = gω2