f=μmg
F-f=ma
F-μmg=ma
加速度a = F/m-μg = 6/2-0.2*10 = 1m/s^2
物体在4秒末的速度v=at = 1*4 = 4m/s
物体在4秒内的位移S = 1/2at^2 = 1/2*1*4^2 = 8m