a
$\mathrm{b}=\frac{\mathrm{F}}{\mathrm{v}}=\frac{120}{1}, \mathrm{r}=\frac{\mathrm{b}}{2 \mathrm{m}}=\frac{60}{7000 \times 2}$
$\omega=\sqrt{\omega_{0}^{2}-r^{2}}$
$\mathrm{kx}=\mathrm{F}$
$\mathrm{k}=\frac{450}{2}=225=\sqrt{\frac{225}{700}-\frac{60^{2}}{700^{2}}}$
$=\sqrt{\frac{225 \times 700-3600}{700^{2}}}=0.56 \mathrm{rad} / \mathrm{s}$