2 Particles, having mass of $\displaystyle m$ and $\displaystyle 2m$ respectively are connected by a light inextensible string, threaded through a fixed smooth ring. The lighter particle moves uniformly around a horizontal circle radius of radius $\displaystyle r$, while the other particle remains at rest.

Find the speed of the lighter particle?

No idea how start this, I've dealt with a bit of circular motion but not sure how to relate it to the mass of the particles?

Thanks in advance for any help