根据动量守恒。mV=(M+m)v1v1=mv/(M+m)=0.2*根号7/(0.8+0.2)=(根号7)/5m/s根据能量守恒1/2mv^2=1/2(m+M)v1^2+fLfL=1/2mv^2-1/2(m+M)v1^2=0.56Jf=0.56/1.4=0.4Nf=umgu=f/mg=0.2
