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

Answer

B

Explanation

Let `x_("max")=`maximum extension of spring
Decrease in potential energy of A
` =` Increase in elastic potential energy of spring
`:. m_(A)gx_("max") =1/2k_("max")_("max")^(2)`
`:. x_("max") = (2m_(A)g)/(k)`
To just lift the block B,
`kx_("max") =m_(B)g`
`:. 2m_(A)g =m_(B)g =mg`
`:. m_(A) =m/2`
.