A block of mass
m is suspended from one end of a light spring as shown. The origin
O is considered at distance equal to natural length of spring from ceiling and vertical downward direction as positive
Y−axis. When the system is in equilibrium a bullet of mass
m/3 moving in vertical upward direction with velocity
v0 strikes the block and embeds into it. As a result, the block (with bullet embedded into it) moves up and starts oscillating.
The amplitude of oscillation would be: