(1)a1=(F-μmg)/m=(10-5)/2=2.5m/s^2
v=a1t=2.5*4=10m/s
(2)s=½a1t^2=0.5*2.5*16=20m
(3)a2=μg=2.5m/s^2=a1
s总=2s=40m