Consider elastic collision of a particle of mass m moving with a velocity u with another particle of the same mass at rest. After the collision the projectile and the struck particle move in directions making angles ${\mathrm{\theta }}_1\text{ and }{\mathrm{\theta }}_2$ respectively with the initial direction of motion. The sum of the angles. ${\mathrm{\theta }}_1+{\mathrm{\theta }}_2$ is