A object of certain mass is placed on a platform which oscillates up and down  simple harmonically. If it is placed on the weighing machine, the reading of the  machine will be

In a simple harmonic motion, the acceleration is always directed opposite to  displacement. Hence at highest point, acceleration will be maximum in downward  direction. So, the pseudo force on the person will be maximum in upward direction.  This makes the normal reaction on the weighing machine decrease and hence  shows minimum reading