球做的运动是自由落体,所以h=1/2gtt,v=gt,h=1/2vt=0.5vv/g,1/2h=0.25vv/g.
设所求为T,则1/2h=1/2gTT,0.25vv/g=0.5gTT,解得：T=根号1/2*v/g