A uniform rod $\mathrm{AB}$ of mass $2 \mathrm{kg}$ and length $1 \mathrm{m}$ is placed on a sharp support $\mathrm{O}$, such that $\mathrm{AO}=a=25 \mathrm{cm}$ and $\mathrm{OB}=b=75 \mathrm{cm}$. A spring of force constant $600 \mathrm{N} {\mathrm{m}}^{-1}$ is attached to the end $\mathrm{B}$, as shown. Initially, the spring is extended by $2 \mathrm{cm}$ at equilibrium. As soon as the thread is burnt, find the normal force (in $\mathrm{N}$) exerted by the support at $\mathrm{O}$.