6 A block, of mass 5 kg , is attached to one end of a length of elastic string. The other end of the string is fixed to a vertical wall. The block is placed on a horizontal surface.

The elastic string has natural length 1.2 m and modulus of elasticity 180 N . The block is pulled so that it is 2 m from the wall and is then released from rest. Whilst taut, the string remains horizontal. It may be assumed that, after the string becomes slack, it does not interfere with the movement of the block.\\
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\begin{enumerate}[label=(\alph*)]
\item Calculate the elastic potential energy when the block is 2 m from the wall.
\item If the horizontal surface is smooth, find the speed of the block when it hits the wall.
\item The surface is in fact rough and the coefficient of friction between the block and the surface is $\mu$.

Find $\mu$ if the block comes to rest just as it reaches the wall.
\end{enumerate}

\hfill \mbox{\textit{AQA M2 2009 Q6 [12]}}