Outlines of natural philosophy Author:John Playfair 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: Sect. III. COMMUNICATION OF MOTION BY IMPULSE. 68. The action and reaction of bodies on one another are equal. This is usually termed the Second Law Op ... more »Motion. When expressed more precisely, it involves two distinct propositions. 1. When motion is communicated by collision or impulse, the quantity of motion gained by the one body in any direction, is just equal to that which is lost by the other in the same direction. 2. When motion is communicated without apparent contact, as in the case of gravitation, and of the phenomena ascribed to attraction or repulsion, the quantity of motion gained by the one body is just equal to that which is gained by the other, but in an opposite direction. The first of these propositions, involves in it the first law of motion, and when so understood, it properly expresses what is called the Inertia Of Body. Like the first law of motion, it follows necessarily from the impenetrability of body. The second proposition is only known to us by experience, The demonstration of the first depends on the following theorem. 69- If two bodies meet with velocities that are inversely as their masses, and directly opposed to one another, there will be an equilibrium. If the bodies are equal, and have equal velocities, the proposition is evident. If A be double of B, but have only half the velocity of B ; then A may be divided into two parts equal to B, each of which will have half the velocity of B. One of these striking against B, would destroy half its velocity ; and the other striking against it, would destroy the other half. The same is true whatever multiple the one body be of the other, and therefore also whatever ratio the one bear to the other. — Vid. D'alembert, Dyna- mique, § 46. p. 51. Hence bodies that have equal ...« less