A bullet of mass $$m$$ moving with velocity $$v$$ is fired into a wooden block of mass $$M$$, If the bullet remains embedded in the block, the final velocity of the system is

A block of mass ' $m$ ' moving on a frictionless surface at speed ' $v$ ' collides elastically with a block of same mass, initially at rest. Now the first block moves at an angle ' $\theta$ ' with its initial direction and has speed ' $v_1$ '. The speed of the second block after collision is