Two block of mass `2kg` are connected by a massless ideal spring of spring contant `K=10N//m` . The upper block is suspended from roof by a light string `A` . The system shown is in equilibrium. The string `A` is now cut, the acceleration of upper block just after the string `A` is cut will be `(g=10m//s^(2)` .

Options

A. `0m//s^(2)`
B. `10m//s^(2)`
C. `15m//s^(2)`
D. `20m//s^(2)`

Answer

D

Explanation

Before the string `A` is cut:
Let `x` be elongation in the spring. As system is in equilibrium. Then for lower block,
`kx=mg=20N`
just after the string `A` is cut:
For upper block,
`ma=kx+mg`
implies `2a=2+20`
implies `a=20m//s^(2)` .