when the smaller block is at the lower extreme from the equilibrium position it has an upward acceleration.as we know that in shm acceleartion is maximum at extreme position.
we can calculate acceleration at extreme position with the formula
so there is upward force of 10N
So the net downward force is 40-10=30N
so it is the minimum force .
If the ball is some where other than extreme position it does apply any force on the block.at that time the down awrd force is due the bigger blocks own weight i.e 40N