Figure $6-39$ is an overhead view of a $12 \mathrm{~kg}$ tire that is to be pulled by three ropes. One force $\left(\vec{F}_{A}\right.$, with magnitude $50 \quad \mathrm{~N}$ ) is indicated. Orient the other two forces $\vec{F}_{B}$ and $\vec{F}_{C}$ so that the magnitude of the resulting acceleration of the tire is least, and find that magnitude if (a) $F_{B}=30 \mathrm{~N}, F_{C}=20 \mathrm{~N} ;$ (b) $F_{B}=30 \mathrm{~N}, F_{C}=10 \mathrm{~N} ;$ and $(\mathrm{c})$ $F_{B}=F_{C}=30 \mathrm{~N}$.