
$F$
$\therefore \mathrm{F}=\mathrm{F}_{\mathrm{AB}}+\mathrm{F}_{\mathrm{BS}}$
$=\mu_{\mathrm{AB}} \mathrm{m}_{\mathrm{A}} \mathrm{g}+\mu_{\mathrm{BS}}\left(\mathrm{m}_{\mathrm{A}}+\mathrm{m}_{\mathrm{B}}\right) \mathrm{g}$
${=0.2 \times 100 \times 10+0.3(100+200) 10}$
${=200+900=1100 \mathrm{N}}$
$(A)$ $\theta=45^{\circ}$
$(B)$ $\theta>45^{\circ}$ and a frictional force acts on the block towards $P$.
$(C)$ $\theta>45^{\circ}$ and a frictional force acts on the block towards $Q$.
$(D)$ $\theta<45^{\circ}$ and a frictional force acts on the block towards $Q$.


