设窗顶距屋檐h距离,则窗底距屋檐为h+2;雨滴自屋檐自由下落,
所以有：h=1/2gt^2;
h+2=1/2g(t+0.4)^2;
两式相减：2=4t+0.8;
t=0.3s;
h=1/2gt^2=0.45m;