MECHANICS AND PHYSICS
TO build up, as did the Greeks, a scientific astronomy which was altogether different from astrology, is a task which presents very great difficulties; but when it is a question of explaining physical and mechanical phenomena, these difficulties become almost insurmountable. In this domain we come into collision with such a variety of aspects that it seems impossible to derive them all from a small number of primary notions.
A badly-hewn tree trunk is in equilibrium on a beam. We feel instinctively that the equal division of the weight round the point of support is the cause of this phenomenon. But how can it be explained accurately? And is the equality of weights the sole cause? A bag of sand placed on a bar of iron can remain in equilibrium even if the sand is not equally distributed on the two sides of the bar.
A piece of deal and a piece of cork of the same size are thrown into the water. The latter sinks less than the former. Is it possible to explain this fact by means of the same theories which make comprehensible the state of equilibrium of the beam or of the bag of sand?
Again, it is quite another matter if we pass from the study of bodies at rest or in equilibrium to the study of bodies in motion. We know that a stone falling freely from the height of a tower accelerates its fall. How is this increase of speed to be accurately measured?