Problem source

$\,$
\vspace*{-0.5in}
\begin{center}
\psset{xunit=1.0cm,yunit=1.0cm,algebraic=true,dimen=middle,dotstyle=o,dotsize=3pt 0,linewidth=0.5pt,arrowsize=3pt 2,arrowinset=0.25}
\begin{pspicture*}(-2.23,-0.31)(9.2,7.46)
\psline(0,0)(8,0)
\psline(0,0)(6.96,2.35)
\psline(1.01,0.34)(-0.61,5.05)
\psline(-0.61,5.05)(5.3,7.1)
\psline(5.3,7.1)(6.96,2.35)
\psline(0.84,0.83)(6.77,2.87)
\psline[linestyle=dashed,dash=2pt 2pt](3,4)(4.16,0.56)
\psline{->}(3,4)(9,4)
\psline[linestyle=dashed,dash=2pt 2pt](3,4)(-1.37,2.42)
\psline[linewidth=0.4pt,linestyle=dashed,dash=2pt 2pt]{->}(-0.43,2.76)(0.43,0.16)
\psline[linestyle=dashed,dash=2pt 2pt]{->}(0.43,0.15)(-0.43,2.75)
\psline[linestyle=dashed,dash=2pt 2pt](6.96,2.35)(7.29,1.39)
\parametricplot{0.0}{0.32693321010422505}{1.28*cos(t)+0|1.28*sin(t)+0}
\psline{->}(4.02,0.97)(7.1,1.93)
\psline{->}(7.1,1.93)(4.02,0.97)
\psline[linewidth=0.4pt,linestyle=dashed,dash=2pt 2pt]{->}(-0.71,5.29)(5.24,7.32)
\psline[linestyle=dashed,dash=2pt 2pt]{->}(5.24,7.32)(-0.71,5.29)
\psline(6.96,2.35)(7.98,2.71)
\rput[tl](2.58,3.67){$G$}
\rput[tl](8.9,3.81){$P$}
\rput[tl](5.64,1.39){$d$}
\rput[tl](1.81,6.79){$2d$}
\rput[tl](-0.59,1.5){$h$}
\rput[tl](1.52,0.32){$\alpha$}
\begin{scriptsize}
\psdots[dotstyle=*](3,4)
\end{scriptsize}
\end{pspicture*}
\end{center}
A lorry of weight $W$ stands on a plane inclined at an angle $\alpha$ to the
horizontal. Its wheels are a distance $2d$ apart, and its centre of
gravity $G$ is at a distance $h$ from the plane, and halfway between the sides
of the lorry. A horizontal  force $P$ acts on the lorry 
through $G\,$, as shown.
\begin{questionparts}
\item If the normal reactions on the lower and 
higher  wheels of the lorry  are equal,
show that the sum of the  frictional forces between the  wheels  and the ground is zero. 
\item If  $P$ is such  that the lorry does not tip
over (but the normal reactions on the lower and 
higher  wheels of the lorry  need not be equal), show that
\[
W\tan(\alpha - \beta)
\le P
\le
W\tan(\alpha+\beta)\;,
\]
where $\tan\beta = d/h\,$.
\end{questionparts}