am=μg=-4m/s,aM=（0.4×0.1×10-（0.2＋0.1）×10×0.1）/0.2=0.5m/s。0.9-4t=0.5t。t=0.2s。x=0.9×0.2-1/2×4×0.2∧2=0.1。不对请指正。
