a) $p=m \nu$ e $p_{i}=p_{f}$ $M \times v_{\text {ônibus }}=(M+m) \times v_{\text {ônibus+carro }}$ $v_{\text {ônibus+carro }}=\frac{M \times v_{\text {ônibus }}}{M+m}=\frac{(9000 \mathrm{~kg}) \times(80 \mathrm{~km} / \mathrm{h})}{9000 \mathrm{~kg}+1000 \mathrm{~kg}}=72 \mathrm{~km} / \mathrm{h}=20 \mathrm{~m} / \mathrm{s}$ b) $F_{L}=F_{a t} \operatorname{sen} 3^{\circ}=8000 \mathrm{~N} \times 0,05=400 \mathrm{~N}$ $a=\frac{F_{L}}{m}=\frac{400 \mathrm{~N}}{1600 \mathrm{~kg}}=0,25 \mathrm{~m} / \mathrm{s}^{2}$