120 km/h=100/3 m/s 【1 m/s=3.6 km/h 】vt²-v0²=2as => 0-10000/9=2a*5 => a=-10000/90=-1000/9 (m/s²）vt=vot+at²/2 => 0=(100/3)t+（-1000/9)t²/2 => t1=0[舍] t2=(100/3)/(1000/18)=0....