4m初动量P=2mc由动量守恒的：2mc=MV又由质能方程得：E=(m-M)c^2=2mc^2-1/2(Mv^2)求得M=4m
应该是5m第一位仁兄回答得很简洁了PS不要认为初动量就是P=2mc
仅供参考。
5m mc^2+2mc^2+(2m)c^2=Mc^2M=5m
