The reference system is used to describe the movements of objects as a standard for other objects. To the same movement, different reference systems have different descriptions. Due to select different reference systems, external force acting or the kinetic energy of an object is different in the kinetic energy theorem and the mechanical energy conservation relations. Thus the establishment of theorem or law or the degree of computation is influenced. Therefore, in general principle, the selection reference system is the inertia system, simply and convenient in the application of kinetic energy theorem or mechanical energy conservation relations.
Key words: theorem of kinetic energy; conservation law of mechanical energy; reference frame
