CHAPTER FOUR
free fall (UFF) but should not be confounded with what certain authors
call the principle of the uniqueness of free fall and also abbreviate by UFF
but use as a synonym for WEP.
12
Our distinction between WEP
kin
and WEP
dyn
is motivated by logical
and historical reasons. WEP
kin
does not presuppose the concept of mass
in any of its meanings and could therefore historically have preceded
WEP
dyn
before the notion of mass was conceived. In fact, WEP
kin
, which
contradicts the Aristotelian thesis that heavy bodies fall faster than light
ones of the same material, can be traced back to the ancient atomists.
Epicurus of Samos, for instance, declared about 300 b.c., in his letter to
Herodotus, that “the atoms must fall with equal velocity (“isotacheis”)
when they are moving through the void.”
13
Similarly, the sixth-century
commentator Ioannis Philoponus, also called John the Grammarian, one
of the early critics of Aristotelian physics, wrote that, “if you let fall from
the same height two weights of which one is many times as heavy as
the other, you will see that the ratio of the times required for the motion
does not depend on the ratio of the weights.”
14
With this statement Philoponus clearly anticipated Galileo Galilei’s
famous, but probably only apocryphal, experiment of dropping two
objects of different weights simultaneously from the top of the Leaning
Tower of Pisa to show that they reach ground at the same time. We
shall not discuss here the question of whether, or how far, the idea
of the experiment had been anticipated by Galileo’s immediate pre-
decessors, and among them especially by Giovanni Battista Benedetti
in his Demonstratio Proportionum Localium (1554). Less known but not
less ingenious was Galileo’s thought experiment, which he designed
“to prove, by means of a short and conclusive argument, that a heavier
body does not move more rapidly than a lighter one provided both
bodies are of the same material.” Galileo imagined a light stone being
attached to a heavy stone. When both are dropped, then according to
Aristotle’s theory the light stone would slow down the heavy stone so
that the combined system would fall more slowly than the heavy stone;
but since the combined system is heavier than the heavy stone alone, it
12
See, e.g., the widely used text by C. W. Misner, K. S. Thorne, and J. A. Wheeler,
Gravitation (San Francisco: Freeman, 1973), p. 1050.
13
P. von der Muehll, ed., Epicuri Epistulae Tres (Letter 1, 61.6) (Stuttgart: Teubner, 1975),
p. 16. See also T. Lucretius, De Rerum Natura Libri Sex, book 2, verse 238–239.
14
Ioannis Philoponi in Aristotelis Physicorum Libros Quinque Posteriores Commentaria
(Berlin: Reimer, 1888), pp. 676–684. English translation in M. R. Cohen and I. E. Drabkin,
A Source Book in Greek Science (New York: McGraw-Hill, 1948), pp. 217–231.
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