[tex]v=108 km/h=30 m/s\\a) v_A=0=>E_{c_A}=0\\E_{p_A}=mgh\\E_{m_A}=E_{c_A}+E_{p_A}=E_{p_A}=mgh\\b)E_{c_B}=\frac{mv^2}{2}\\c)\Delta E_{c_{AB}}=L_{G_{AB}}+L_{F_f{_{AB}}}+L_{N_{AB}}\\L_{N_{AB}}=0\\L_{G_{AB}}=mgh\\\Delta E_{c_{AB}}=E_{c_B}-E_{c_A}=E_{c_B}=\frac{mv^2}{2} \\\Delta E_{c_{AB}}=L_{G_{AB}}+L_{F_f{_{AB}}}+L_{N_{AB}}<=>\frac{mv^2}{2}=mgh+L_{F_f{_{AB}}}=>L_{F_f{_{AB}}}=\frac{mv^2}{2}-mgh=m(\frac{v^2}{2}-gh)=>L_{F_f{_{AB}}}=m(\frac{v^2}{2}-gh)\\[/tex]
[tex]d)L_{F_f{_{AB}}}=-F_f*AB=-\mu mgcos\alpha*AB<=>m(\frac{v^2}{2}-gh)= -\mu mgcos\alpha*AB\\cos\alpha*AB=l\\m(\frac{v^2}{2}-gh)= -\mu mgl|:m=>\mu gl=gh-\frac{v^2}{2}=\frac{2gh-v^2}{2}=>\mu=\frac{2gh-v^2}{2gl}[/tex]
