牛顿第三定律在相对论条件下一般不成立

在一般的大学物理,电动力学或狭义相对论等教科书中,只是简单地指出:当物体运动速度可与真空中光速相比拟时,牛顿定律不再成立。或进一步指出:牛顿第二定律 F=ma 需要用新的动力学方程 F=dP/dt 来取代。至于牛顿第三定律在相对论条件下是否成立,一般都不加阐述。本文从相对论的时空观出发,进行定量分析,得出牛顿第三定律在相对论条件下一般不成立。只有当相互作用的两物体相互接触,而且相对速度为零时,牛顿第三定律才严格成立。当然,这并不排斥牛顿定律的重要性。它仍然是经典力学的基本定律,而且是在低速情况下相对论力学的一种极好近似。对于工程技术上存在的大量力学问题,牛顿力学还是足够精确的。

NEWTON'S THIRD LAW IS COMMONLY NOT VALID IN RELATIVITY THEORY

In almost all the textbooks of university physics,electro-dynamics and special relativity,it is only pointed out that Newton's laws are not valid when the speed of a body can be compared with that of light.Sometimes,it is also pointed out that the equation of Newton's second law F=ma,must be replaced by a new dynamic equation F=dP/dt.Generally,the effectiveness of Newton's third law in the case of relativity is not illustrated. In this paper the quantitative analysis is carried out in terms of time-space view- point of relativity,and it follows that Newton's third law is commonly not valid in the case of relativity.Only when two acting bodies contact each other,and the relative velocity is equal to zero,Newton's third law is exact.But Newton's laws are still very important,and they are the fundamental laws in classical mechanics and perfect approximate of relativistic mechanics under the condition of low speed. Newton's mechanics is exact enough for almost all the problems in engineering and technology domains.