Give a reason why the model used for the resistance to motion may not be realistic.
\section*{3.}
\section*{Figure 1}
\includegraphics[max width=\textwidth, alt={}]{19f831ad-5e32-470c-9974-beb82d5c9753-3_751_678_440_657}
A uniform ladder \(A B\), of mass \(m\) and length \(2 a\), has one end \(A\) on rough horizontal ground. The other end \(B\) rests against a smooth vertical wall. The ladder is in a vertical plane perpendicular to the wall. The ladder makes an angle \(\alpha\) with the horizontal, where \(\tan \alpha = \frac { 4 } { 3 }\). A child of mass \(2 m\) stands on the ladder at \(C\) where \(A C = \frac { 1 } { 2 } a\), as shown in Fig. 1. The ladder and the child are in equilibrium.
By modelling the ladder as a rod and the child as a particle, calculate the least possible value of the coefficient of friction between the ladder and the ground.