Question

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

• A. 2M
• B. $\frac{M}{2}$
• C. M
• D. cannot say
  

Answer 1

The correct option is B $\frac{M}{2}$

The spring will exert maximum force when A is at lowest position, say at distance x from the present position.
$\therefore mgx=\frac{1}{2}k{x}^2$
Where, k is force constant of the spring, for the block B to leave contact, kx = Mg
$\therefore mgx=\frac{1}{2}Mgx\therefore m=\frac{M}{2}$