${\mathrm{u}=0, \mathrm{t}=2 \mathrm{s}, \mathrm{s}=8 \mathrm{m}, \mathrm{a}=?} $

${\mathrm{S}=\mathrm{ut}+\frac{1}{2} \mathrm{at}^{2}}$

$\text { where }  a=\operatorname{gsin} 30^{\circ}-\mu g \cos 30^{\circ} $

$=\frac{g}{2}[1-\sqrt{3} \mu] $

$\text { Thus }  8=0+\frac{1}{2} \frac{g}{2}(1-\sqrt{3} \mu) \times 4$

$ 1-\sqrt{3} \mu =\frac{8}{\mathrm{g}}=\frac{8}{10}=\frac{4}{5} $

$ \sqrt{3} \mu =1-\frac{4}{5}=\frac{1}{5} $

$\mu =\frac{1}{5 \sqrt{3}} $