The position and velocity of a particle executing simple harmonic motion at $t=0$ are given by $\mathrm{3 }\mathrm{cm}$ and $\mathrm{8 }\mathrm{cm} {\mathrm{s}}^{-1}$ respectively. If the angular frequency of the particle is $2 \mathrm{rad} {\mathrm{s}}^{-1}$, then the amplitude of oscillation (in $\mathrm{cm}$)  is