A body is projected at $t=0$ with a velocity $10 \mathrm{~ms}^{-1}$ at an angle of $60^{\circ}$ with the horizontal. The radius of curvature of its trajectory at $t=1 \mathrm{~s}$ is $R$. Neglecting air resistance and taking acceleration due to gravity $g=10 \mathrm{~ms}^{-2}$, the value of $R$ is