Philosophy and the New Physics Author:Louis Auguste Paul Rougier Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER II MASS AND THE RELATIVITY PRINCIPLE 6. THE IDEA OF MASS; EINSTEIN's EQUIVALENCE PRINCIPLE AND NEWTON 's PRINCIPLE OF ACTION AND REACTION. In order... more » to show how the idea of energy as endowed with mass and weight in proportion thereto has been arrived at, it is necessary to define these two ideas and to state two principles which are closely associated with them. What constitutes the individuality of a piece of matter through all motions, divisions, recomposi- tions, compressions, expansions, changes of state and chemical combinations that it undergoes, is its mass, which presents itself as an invariant of the group of physico-chemical changes.1 It is considered in classical mechanics as an invariable scalar quantity that characterizes every piece of matter and may be defined in three different ways: (1) as a coefficient of inertia; (2) as capacity for momentum; (3) as capacity for kinetic energy. As coefficient of inertia, mass measures the resistance of a body to any action tending to modify its state of motion. Newton assumes the proportionality of the force acting on a body to the change in velocity per unit time communicated to it, or the acceleration; therefore the quotient of force by acceleration defines the mass of a body. Moreover, he assumes the principle of the independence of the effects of a force, which leads to the view that the mass of a moving body is independent of the velocity acquired: if a force acts on a body for a second, starting from rest, and communicates to it the velocity v, the same force acting through another second will communicate to it a second increment of velocity equal to the first, so that its velocity will become 2v; if the same force continues to act through a third second, the velocity will become 3i and so on. Theoretica...« less