(a) Angular velocity $= \omega$
Centripetal force $F = mr\omega^{2}$
or $r \propto \frac{1}{\omega^{2}}$
$\therefore$ $\frac{r_{1}}{r_{2}} = \frac{\omega_{2}^{2}}{\omega_{1}^{2}}$
or $\frac{4}{r_{2}} = \frac{4\omega^{2}}{\omega^{2}}$
or $r_{2} = 1$ cm