Discrete random variables — practice question

A light elastic string has natural length $a\text{ m}$ and modulus of elasticity $\lambda\text{ N}$. Its tension is $4\text{ N}$ when the length is $1.6\text{ m}$, and it is $6\text{ N}$ when the length is $2\text{ m}$. One end of the string is fixed at a point $O$ on a smooth horizontal surface, while the other end is connected to a particle $P$ of mass $0.2\text{ kg}$. The particle $P$ moves at constant speed on the surface in a circle with centre $O$ and radius $1.9\text{ m}$.