$$\left\{\begin{array}{cccc} r_1=0.02\,\rm{m} \\ m_1=0.01\,\rm{kg} \\ N=600\,\rm{rpm} \\ r_2=0.015\,\rm{m} \end{array}\right.$$ $(a)$ $$\begin{align} 遠心力 : F&=m_1r_1\omega^2 \\ &=0.01 \,\rm{kg} \times 0.02 \,\rm{m} \times \left( \frac{2 \pi \times 600 \,\rm{rpm} }{60} \right)^2 \\ &=0.7895 \,\rm{N} \\ \end{align}$$ $$\therefore F=0.790 \ \,\rm{N}$$
$(b)$ $$m_1r_1\omega^2 = m_2r_2\omega^2 \\ m_1r_1 = m_2r_2 \\ \begin{align} m_2 &=\frac{r_1}{r_2}m_1=\frac{0.02 \,\rm{m} }{0.015 \,\rm{m}} \times 0.01 \,\rm{kg}\\ &=0.01333 \,\rm{kg} \\ \end{align}$$ $$\therefore m_2 = 13.3\,\rm{g}$$