到最高点时物体速度为零 v=0,
0=40-10t²
t=2
s = [0,2 ] ∫ vdt = [0,2 ] ∫ (40-10t²)dt = (40t - 10t³/3) |[0,2 ] = 80-80/3 = 160/3 ≈ 53.3 s