已知向量 $\stackrel{\rightharpoonup }{AB}$与 $\stackrel{\rightharpoonup }{AC}$的夹角为 ${120}^{°}$，且 $\left|\stackrel{\rightharpoonup }{AB}\right|=3,\left|\stackrel{\rightharpoonup }{AC}\right|=2$,若 $\stackrel{\rightharpoonup }{AP}=\lambda \stackrel{\rightharpoonup }{AB}+\stackrel{\rightharpoonup }{AC}$,且 $\stackrel{\rightharpoonup }{AP}\perp \stackrel{\rightharpoonup }{BC}$,则实数 $\lambda$的值为.