The sand on a platform that is oscillating with a certain frequency and amplitude will soon find itself becoming airborne if the acceleration approaches 9.81 ms-2. Taking the position of the platform at the highest point (max displacement) , the acceleration a is the greatest,but the only force on the sand is weight (down) and normal reaction(up). Therefore , writing the summation of force = ma , mg-R = ma, the max possible a is g , i.e when R=0 , the sand will become airborne. Equate g to the acceleration due to shm , you can calculate the max. amplitude for a given f or max f for a given amplitude.
It may be easy to imagine yourself on a trampoline that bounces with shm, is it likely that you become airborne sometime .Isn't it obvious where you will become airborne. To analyse such cases, always draw free body diagram and recognising that the acceleration always point towards the equilibrium. At the trough of the motion, the reaction force is always greater that the weight.
