Lecture I

Let us suppose that we have here a mountain or hill, and beside it, a steep slope. On the slope there is a spring and the flow from it leaps sheer down, a real waterfall. Higher up on the same slope is another spring; the water from it would like to leap down in the same way, but it does not. It cannot behave as a waterfall, but runs down nicely as a stream or beck. Is the water itself endowed with different forces in these two cases? Quite clearly not. For the second stream would behave just as the first stream does if it were not obstructed by the shape of the mountain. If the obstructive force of the mountain were not present, the second stream would go leaping down. Thus we have to reckon with two forces: the obstructive force of the mountain and the earth's gravitational pull, which turns the first stream into a waterfall. The gravitational force acts also on the second stream—one can see how it brings the stream flowing down. But a skeptic could say that in the case of the second stream this is not at all obvious, whereas in the first stream every particle of water goes hurtling down. In the case of the second stream we must reckon in at every point the obstructing force of the mountain, which acts in opposition to the earth's gravitational pull.