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

Options

A. `2M`
B. `M`
C. `M//2`
D. A function of `M` and the force constant of the spring.

Answer

C

Explanation

The spring will exert the maximum force when the ball is at its lowest postion. If the ball has descended through a distance `x` to reach this position, `mgx=(1)/(2)kx^(2)`. Where `k` is the force constant of the spring. For the block `B` to leave contact, spring force `=kx=Mg`