(if it's frictionless the length doesn't even matter :) )
It would have the same kinetic energy down as the potential energy up. That is, [tex]mgh=\frac{mv^2}{2}[/tex] or [tex]2gh=v^2[/tex] (the mass doesn't even matter). The result is [tex]\sqrt{2gh}[/tex], so only the height matters really. It is almost 9 (it is [tex]\sqrt{80}=4\sqrt{5}[/tex]).
