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

By Victor F. Lenzen | Go to book overview

CHAPTER IV
KINEMATICS

1. KINETICS OF A POINT

IN the preceding chapters we have considered the concepts of geometrical properties, especially the concepts of geometrical quantities such as the measure of distance, angle, etc., and further the concepts of temporal properties and of temporal quantities such as the measure of time interval. In the general sense these are all physical quantities: they are measure numbers which are assigned to measurable characters. The quantities are referred to as: the distance, the angle, the x coordinate, the time. In the present chapter we shall take up the concepts of quantities which serve to describe motion, The subject matter of this chapter is kinematics, which may be characterized as the description of motion. Inasmuch as motion is change of position in space with time, the quantities of kinematics are to be defined in terms of geometrical and temporal quantities.

The fundamental condition for the description of motion is the choice of a frame of reference. The representation of the motion of any body depends upon the frame of reference employed, as can be seen from a simple example. Suppose that a train is moving along a straight track, and a man in the train drops a stone out of the window. What is the path of the stone? The man in the train would use the car as a frame of reference, and relative to this frame the stone moves in a straight line. An observer on the ground would choose the surface of the earth as a frame of reference, and relative to it the stone moves in a parabolic path. Relative to a frame attached to the sun the path would be still different. Thus the description of motion presupposes the selection of a frame

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