虚位移之“虚”表现何在?

What does “virtual”mean in the virtual displacement?

阐述使用虚位移处理具体问题可以分解为三步:1)对一个具体的物理问题,给出自变量虚位移的定义;2)对这个问题需要的函数,由于自变量的虚位移产生的变化;3)对相关函数加上物理要求.经过这三步之后,才能给出物理上合理的结果.而其中的第二步,体现了其非真实性.

Virtual displacement is an important physical concept in physics. For exaggerating the role of the non-reality in applications of the virtual displacement,we analyze how the virtual displacement is applied,which can be divided into three steps： 1） for a given physical problem,the definition of virtual displacement of a variable is introduced; 2） the variation of a mathematical function along the change of the virtual displacement is given,which is not necessarily compatible with physics principles; 3） with imposing physical conditions on the mathematical function,the physics consequence is resulted.