A Treatise: On Elementary Dynamics for the Use of Colleges and Schools

Excerpt from A Treatise: On Elementary Dynamics for the Use of Colleges and Schools

1. A point is said to be in motion when it changes its position relative to surrounding objects.

From this definition it will be seen that all cases of motion which will come under our consideration are essentially relative; in fact, we have no means of measuring absolute motion, or of determining whether any given point is absolutely at rest in space, or not, and it may even be doubted whether the human mind is capable of forming a distinct conception of absolute motion.

We shall not attempt to give a definition of space; our idea of it must be considered a primary conception.

Time is defined by the metaphysician as "the succession of ideas"; the physicist treats time, like space, as a primary conception.

Equal times are generally defined as those intervals during which the earth turns through equal angles relative to the fixed stars, and any duration of time may then be measured by the angle turned through by the earth during the interval. The most obvious unit of time is therefore the sidereal day, or the period during which the earth makes a complete rotation on its axis relative to the fixed stars. The unit generally adopted is the second of mean solar time.

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