The Nature of Physical Theory: A Study in Theory of Knowledge

By Victor F. Lenzen | Go to book overview

CHAPTER V
DYNAMICS

1. INTRODUCTION

WE have seen how one can describe motion in terms of the concepts of space and time. Now the sort of motion that occurs depends on the characters of the moving body and of external bodies. Dynamics is the study of the conditions of motion. In order to describe the conditions of motion it is necessary to add to the concepts of the geometrical and temporal quantities employed in kinematics two new fundamental concepts: those of force and mass. The physical quantity mass is assumed to represent some intrinsic character of a body. Force is often defined as the cause of changes of motion, and dynamics is said to be the study of the action of force upon material bodies. If kinematics is characterized as the description of motion, dynamics may be characterized as the description of the conditions of motion. Thus one may say that dynamics offers an explanation of motion, meaning by explanation, however, the determination of empirical conditions.

The first step in the construction of dynamics is the selection of a frame of reference. In classical dynamics one chooses as a first approximation a rigid frame attached to the earth. One may then define concepts and discover laws which very accurately determine the motions of bodies with respect to the earth. However, there are motions with respect to the earth which do not conform to the laws of dynamics, such as the rotation of the plane of vibration of a pendulum, the deflection of the trade winds to the right in the northern hemisphere, the difference in the weight of a body at the poles and the equator other than that due to the difference in distance from the center of the earth, etc. One may account for these facts

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