$\frac{\lambda }{2}=20cm\Rightarrow \lambda =40cm=0/4m$ $f=\frac{\omega }{2\pi }=\frac{40\pi }{2\pi }=20Hz$ $\lambda =\frac{v}{f}\Rightarrow 0/4=\frac{v}{20}\Rightarrow v=8\frac{m}{s}$ $\mu =\frac{m}{L}=\frac{100\times {{10}^{-3}}}{0/5}=0/2\frac{kg}{m}$ $v=\sqrt{\frac{F}{\mu }}\Rightarrow 8=\sqrt{\frac{F}{0/2}}\Rightarrow F=12/8N$