(1)计算停下时间T
0=v0+aT
0=5-0.4T
因为T=12.5s〈20s
所以0-v0^2=2ax
-25=-0.8x
x=31.25m
(2)s=v0t+1/2at^2
30=5t+0.2t^2
t=5s