\[\left\{ \begin{array}{c} N{R\cos}^2\alpha -mgR\sin\alpha \cos\alpha +\mu NR\sin\alpha \cos\alpha =0 \\  -\mu N\cdot l{\sin}^2\alpha +mgR\sin\alpha \cos\alpha +NR{\sin}^2\alpha =0 \end{array} \right\]