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

In a test experiment on a model aeroplane in wind tunnel, the flow speeds on the upper and lower surfaces of the wings are $$70 \mathrm{~ms}^{-1}$$ and $$65 \mathrm{~ms}^{-1}$$ respectively. If the wing area is $$2 \mathrm{~m}^2$$, the lift of the wing is _________ $$N$$.

(Given density of air $$=1.2 \mathrm{~kg} \mathrm{~m}^{-3}$$)