A particle of mass m with an initial velocity $u\hat{\text{i}}$ collides perfectly elastically with a mass 3 m at rest. It moves with a velocity $v\hat{\text{j}}$ after collision, then, $v$ is given by

(1) $v=\sqrt{2\over3}u$

(2) $v={u\over\sqrt{3}}$

(3) $v={u\over\sqrt{6}}$

(4) $v={u\over\sqrt{2}}$

(1) $v=\sqrt{2\over3}u$

(2) $v={u\over\sqrt{3}}$

(3) $v={u\over\sqrt{6}}$

(4) $v={u\over\sqrt{2}}$