$T=\frac{\Delta t}{N}=\frac{60}{30}=2s$                    $T=\frac{2\pi r}{v}\Rightarrow 2=\frac{2\pi r}{4}\Rightarrow r=\frac{4}{\pi }m$ ${{a}_{c}}=\frac{{{v}^{2}}}{r}=\frac{4\times 4}{\frac{4}{\pi }}=4\pi \frac{m}{{{s}^{2}}}\simeq 12\frac{m}{{{s}^{2}}}$ ${{F}_{net}}=ma=0/5\times 12=6N$