$\omega =\sqrt{\frac{g}{L}}\Rightarrow {{\omega }^{2}}=\frac{g}{L}=\frac{10}{1/2}=\frac{25}{3}{{(\frac{rad}{s})}^{2}}$                    $a=-{{\omega }^{2}}x\Rightarrow \left| a \right|={{\omega }^{2}}\left| x \right|=\frac{25}{3}\times \frac{3}{100}=\frac{1}{4}\frac{m}{{{s}^{2}}}$