A ball rolls off the top of a stairway with horizontal velocity $$u$$. The steps are $$0.1$$ m high and $$0.1$$ m wide. The minimum velocity $$u$$ with which that ball just hits the step 5 of the stairway will be $$\sqrt{x}$$ m s$$^{-1}$$, where $$x =$$ _______ [use $$g = 10$$ m s$$^{-2}$$].