On a frictionless surface, a block of mass M moving at speed v collides elastically with another block of same mass M which is initially at rest.…

On a frictionless surface, a block of mass $\displaystyle M$ moving at speed $\displaystyle v$ collides elastically with another block of same mass $\displaystyle M$ which is initially at rest. After collision the first block moves at an angle $\displaystyle \theta$ to its initial direction and has a speed $\displaystyle \frac{v}{3}$. The second block's speed after the collision is :