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

Options

A. 3M
B. `(M)/(2)`
C. M
D. cannot say

Answer

B

Explanation

The spring will exert maximum force when A is at lowest position, say at distance x from the present position.
`therefore" "mgx=(1)/(2) kx^(2)`
where, k is force constant of the spring, for the block B to leave contact, kx = Mg
`therefore " "mgx=(1)/(2) Mgx`
`therefore" "m=(M)/(2)`