已知向量$\stackrel{\rightharpoonup }{a},\stackrel{\rightharpoonup }{b}$满足$\left|\stackrel{\rightharpoonup }{a}\right|=1,\left|\stackrel{\rightharpoonup }{b}\right|=2,\stackrel{\rightharpoonup }{a}$与$\stackrel{\rightharpoonup }{b}$的夹角为${60}^{°}$，则$\left|\stackrel{\rightharpoonup }{a}-\stackrel{\rightharpoonup }{b}\right|=$.