设椭圆 $\frac{x^2}{25}+\frac{y^2}{16}=1$上一点 $P$到左准线的距离为10， $F$是该椭圆的左焦点，若点 $M$满足 $\stackrel{\rightharpoonup }{OM}=\frac{1}{2}\left(\stackrel{\rightharpoonup }{OP}+\stackrel{\rightharpoonup }{DF}\right)$，则 $\left|\stackrel{\rightharpoonup }{OM}\right|$ $=$．