a
\(\begin{array}{l}
\,\,\,\,\,\,\,\,Force\,of\,friction,\,f = \mu mg\\
\therefore \,\,a = \frac{f}{m} = \frac{{\mu mg}}{m} = \mu g = 0.5 \times 10 = 5\,m{s^{ - 2}}\\
{\rm{Using}}\,{v^2} - {u^2} = 2as\\
\,\,\,\,\,\,\,\,\,\,{0^2} - {2^2} = 2\left( { - 5} \right) \times S \Rightarrow S = \,0.4\,m
\end{array}\)