A particle is projected with a velocity $u$ making an angel $\theta$ with the horizontal. At any instant its velocity becomes $v$ which is perpendicular to the initial velocity $u$. Then $v$ is

A ball is projected with velocity u making an angle $\theta$ with the horizontal. The magnitude of velocity at the instant when it is perpendicular to its initial velocity of projection

A particle projected from point O with velocity u in a direction making an angle $\alpha$ with the horizontal. At any instant its position is at point P at right angles to the initial direction of projection. Its velocity at point O is