Gravitational and Inertial Mass

One of the most astonishing features of the history of physics is the confusion which surrounds the definition of the key term in dynamics—mass, a confusion which has existed almost from the day that Newton defined it. Originally defined as a measure, i.e., a number, it became something which a body had, which could be measured. Mach considered the definition “unfortunate, ” Einstein “illusory,” and Sommerfeld called it a “mock definition.” Yet without it Newton could not have laid the foundations of dynamics, succeeding where Galileo failed. A careful examination of the problem shows, with very little doubt, that when Newton used the word density he meant what we now call relative density. His definition of mass is then unexceptionable. What is unfortunate is that he allowed himself to speak of mass with the common meaning as well as with the special meaning he had given it. Recently the terms gravitational and inertial mass have been coming into greater use, but the confusion has increased rather than diminished. The author's theory of inertia, which is in quantitative agreement with experiment, helps to lessen the confusion, both old and new.