设总高度为h,下落时间为t
h=(1/2)at^2
依题意有
(1/2)a(t-1)^2=(1/2)h
(1/2)a(t-1)^2=(1/4)at^2
(t-1)^2=2t^2
(t+1)^2=2
t=（根号2）-1
t=-（根号2）-1 （舍去）