[tex]F_f=\frac{G}{8}\\N-G_n=0=>N=mgcos\alpha\\F_f=\mu N=\mu mgcos\alpha\\F_f=\frac{G}{8}<=>\mu mgcos\alpha=\frac{mg}{8}|:mg=>\mu cos\alpha=\frac{1}{8}=>\mu=\frac{1}{8cos\alpha}\\sin\alpha=\frac{h}{l}=\frac{10}{15}=0,666\\ \eta=\frac{Gh}{Fl}=\frac{Gh}{(Gsin\alpha+\mu Gcos\alpha)l}=\frac{Gh}{G(sin\alpha+\mu cos\alpha)l}=\frac{sin\alpha}{sin\alpha(1+\mu\frac{cos\alpha}{sin\alpha})}=\frac{1}{1+ctg\alpha\mu}=\frac{1}{1+\frac{cos\alpha}{sin\alpha}\frac{1}{8cos\alpha}}=\frac{1}{1+\frac{1}{0,666*8}}[/tex][tex]=\frac{1}{1+0,18}=\frac{1}{1,18}=0,84=>\eta=84\%[/tex]
