and
' are the factors of proportionality, and are called the masses of the two bodies, respectively. As
has the same value in both equations, we have
and we can accordingly measure the masses of two bodies at the same place, by determining their weights.
Although Galilei and Newton had already known that all bodies at the same place fall with the same velocity (if the resistance of the air be eliminated), this very remarkable fact has not received any recognition in the foundations of mechanics. Einstein's principle of equivalence is the first to assign to it the position to which it is, beyond doubt, entitled.
[Note 19] (p. 41). Arguing along the same lines B. and J. Friedländer have suggested an experiment to show the relativity of rotational motions, and, accordingly, the reversibility of centrifugal phenomena ("Absolute and Relative Motion," Berlin, Leonhard Simion, 1896). On account of the smallness of the effect, the experiment cannot, at present, be performed successfully; but it is quite appropriate for making the physical content of this postulate more evident. The following remarks may be quoted: