Two blocks A and B of mass `m` and `2m` are connected together by a light spring of stiffness `k`. The system is lying on a smooth horizontal surface with block A in contact with a fixed vertical wall as shown in the figure. The block B is pressed towards the wall by a distance `x_0` and then released. There is not friction anywhere. If spring takes time `Deltat` to aquire its natural length then average force on the block A by the wall is

Explanation

from `COM`
`mv_(A)=2mv_(B)`
`rArr v_(B)=(v_(A))/(2)`
from `COE`
`(1)/(2)kx_(0)^(2)=(1)/(2)mv_(A)^(2)+(1)/(2)xx2m(v_(A)^(2))/(4)=(1)/(2)mv_(A)^(2)(1+(1)/(2))`
`rArr(1)/(2)kx_(0)^(2)=(1)/(2)mv_(A)^(2)xx(3)/(2)rArr(1)/(2)mv_(A)^(2)=(kx_(0)^(2))/(3)`