Problem 1.13

Point $A$ moves uniformly with velocity $v$ so that the vector $\mathbf{v}$ is continually "aimed" at point $B$ which in its turn moves rectilinearly and uniformly with velocity $u < v$. At the initial moment of time $\mathbf{v} \perp \mathbf{u}$ and the points are separated by a distance $l .$ How soon will the points converge?