However, with the development of social economy, the issue of wide gap between the rich and the poor becomes more and more obvious.

Starting from an example, it is explained that the frame of center of mass is more intuitive, simple and reliable compared to other frames in approximate disposal of two-body system with great disparity in mass.

A study of the wide gap between the rich and the poor in China in the light of Gini coefficient;

So much resistance of the common people and appeal of some persons with breadth of vision of the landlord class in ancient China proved such a historical fact: the survival crisis from wide gap between the rich and the poor was the direct source to arouse political crisis.