A system consisting of two blocks of masses m1  and  m2 connected by a spring of spring constant K. A force  F is applied on  m1 along its weight and removed such that  m2 is just lifted. Then F is

For m1  :  F+m1g=kx0                       For m2  :   m2 to lift                           kx=m2g→2 Loss of potential energy  of spring= gain of potential energy of blocks12k(x0−x2)=m1g(x0+x) 12k(x0−x)=m1g kx0−kx=2m1g F+m1g−m2g=2m1g F=(m1+m2)g F=(m1+m2)g