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 the ground at some stage, the minimum mass of A must be
M2
For B
Just to loose contact
Mg=Kx⇒x=MgK
Applying WET
WEe+WDs=ΔKE⇒−12Kx2+mgx=0⇒m=M2