4.

\begin{figure}[h]
\begin{center}
  \begin{tikzpicture}[>=latex, scale=1.5]
    % Configuration variables
    \def\angle{30}      % Inclination angle alpha
    \def\cpos{4.5}      % Distance of point C from origin
    \def\apos{7.0}      % Distance of point A from origin
    \def\totalLen{8.5}  % Length of the incline line

    % 1. Draw the horizontal dashed baseline
    \draw[dashed] (0,0) -- (6,0);

    % 2. Draw the main inclined plane line
    \draw (0,0) -- (\angle:\totalLen);

    % 3. Draw the angle arc and label it alpha
    \draw (1.2, 0) arc (0:\angle:1.2);
    \node at (15:1.5) {$\alpha$};

    % 4. Elements on the incline (using a rotated scope for simplicity)
    \begin{scope}[rotate=\angle]
        % Coordinate definitions
        \coordinate (C) at (\cpos, 0);
        \coordinate (A) at (\apos, 0);

        % Draw the solid dot at C
        \fill ($(C)+(0,.075)$) circle (2pt);
        
        % Draw the perpendicular tick mark at A
        \draw (A) ++(0, 0.12) -- ++(0, -0.24);

        % Draw the thin rod slightly above the incline line
        \draw[thin] (\cpos, 0.08) -- (\apos, 0.08);

        % Draw the double-headed dimension arrow for 'l' below the incline
        \draw[<->] (\cpos, -0.4) -- (\apos, -0.4) node[midway, fill=white, inner sep=1pt] {$l$};

        % Place labels C and A below their respective points
        \node[below=14pt] at (C) {$C$};
        \node[below=10pt, right=6pt] at (A) {$A$};
    \end{scope}

\end{tikzpicture}
\captionsetup{labelformat=empty}
\caption{Figure 3}
\end{center}
\end{figure}

One end of a light elastic string, of natural length $l$ and modulus of elasticity $3 m g$, is fixed to a point $A$ on a fixed plane inclined at an angle $\alpha$ to the horizontal, where $\sin \alpha = \frac { 3 } { 5 }$ A small ball of mass $2 m$ is attached to the free end of the string. The ball is held at a point $C$ on the plane, where $C$ is below $A$ and $A C = l$ as shown in Figure 3. The string is parallel to a line of greatest slope of the plane. The ball is released from rest. In an initial model the plane is assumed to be smooth.
\begin{enumerate}[label=(\alph*)]
\item Find the distance that the ball moves before first coming to instantaneous rest.

In a refined model the plane is assumed to be rough. The coefficient of friction between the ball and the plane is $\mu$. The ball first comes to instantaneous rest after moving a distance $\frac { 2 } { 5 } l$.
\item Find the value of $\mu$.
\end{enumerate}

\hfill \mbox{\textit{Edexcel M3 2014 Q4 [11]}}