# A particle is projected from a point $O$ with velocity $u$ at an angle of $60^{\circ}$ with the horizontal. When it is moving in a direction at right angle to its direction at O, then its velocity is given by :
( A ) $\frac{2u}{3}$
( B ) $\frac{u}{\sqrt{3}}$
( C ) $\frac{u}{3}$
( D ) $\frac{u}{2}$