In the figure ball A is released from rest when the spring is at its natural length. For the block B of mass M to leave contact with ground at some stage, the minimum mass of A must be

Suppose when maximum displacement of the ball is x, when the mass M loses contact with the  ground.∴   kx=Mg  ........(1)Again,  12 kx2−mgx  =0  ⇒  x=2mgk  .......(2)∴   k 2mgk=Mg  ⇒   m=M/2