摩托车由静止开始走完一段长为218m直道。已知摩托车先以4m/s2加速度启动，加速一段时间后再以8m/s2加速度制动，已知摩托车在直道中的最大速度不能超过40m/s，离开直道时的速度为20m/s。求摩托车在直道上行驶所用的最短时间。
解：设摩托车最大速度vm(不一定是40m/s)，加速时间为t1，通过位移为s1；减速时间为t2，减速位移为s2
选速度方向为正方向，则加速时加速度a1＝4m/s2，减速时加速度a2＝－8m/s2
加速时间为
加速位移为
减速时间为
减速位移为
已知
代入数据可得
所以最短的时间为
| 完型填空。 | |||
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