Probability — practice question

Particle $P$, with mass $0.3\,\text{kg}$, is moving round a circle centred at $O$ on a smooth horizontal surface. $P$ is linked to $O$ by a light elastic string whose modulus of elasticity is $12\,\text{N}$ and whose natural length is $l\,\text{m}$. The speed of $P$ is $4\,\text{m s}^{-1}$, and the circle it describes has radius $2l\,\text{m}$.