它落下一半高度时,下落高度 h1 = 19.6 m 由 h1 = 1/2 gt1² 解得 ：t1 = √2h1/g = √2×19.6/9.8 s = 2 s物体下落总高度 H = 1/2 gt² ,解得下落总时间 t = √2H/g = √2×39.2/9.8 s = 2√2 s所以 ,...