已知向量 $\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b}$为不共线的单位向量, $k\in R$,如果 $\left(\stackrel{\rightharpoonup }{a}+\stackrel{\rightharpoonup }{b}\right)与\left(k\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b}\right)$垂直,那么 $k=$.