关于物体系机械能的讨论

物体系的机械能的变化与其非保守内力和非保守外力的相关。在非保守外力不为零时，非保守外力的功随不同的惯性系而变化。因此物体系的机械能在某一惯性系中守恒时，在其他惯性系中不一定守恒。只有在两个惯性系相对运动的速度方向与物体系所受的非保守外力的方向垂直时，物体系的机械能在两个惯性系中才都守恒。

A discussion about the mechanical energy of the objects system

The mechanical energy of the objects system changes with the works of the non con servative inside force and outside force. When the non conservative outside force is not zero, its works depend on the given inertial system. The mechanical energy of the objects system is constant in a inertial system, but maybe not so in the other inertial systems. Only when the direction of the relative velocity between two inertial system is perpendicular to the direction of the non conservative outside force which acts on the objects system, the mechanical energy in the two inertial systems are both constant.