In the figure, block A is released from rest when the spring is its natural length for the block B of mass m to leave contact with the ground at some stage what should be the minimum mass of block A?
.

Options

A. (a) `2M`
B. (b) `M`
C. (c) `M//2`
D. (d) `M//4`

Answer

C

Explanation

The spring will exert maximum force when the ball is at its lowest position. If the ball has descended through a distance x to reach the position,
`mgx=(1//2)Kx^2` or `x=2mg//K` (i)
For the block B to leave contact spring force,
`Kx=Mg` (ii)
Comparing Eqs. (i) and (ii), `m=M//2`