由x=（1/2）atˆ2得a=2x/tˆ2=(2*0.4m)/(1s)ˆ2=0.8m/sˆ2第一秒末的速度v=at=0.8m/sˆ2*1s=0.8m/s前2秒位移x'=（1/2）at'ˆ2=（1/2）*0.8m/sˆ2*(2s)ˆ2=1.6m第二秒内通过的位移x'...