Galileo dropped a cannonball and a
musketball simultaneously from a
tower, and observed that they hit the
ground at nearly the same time. This
contradicted Aristotle’s long-accepted
idea that heavier objects fell faster.
Galileo and the Church
Galileo’s contradiction of Aristotle had serious consequences. He was interrogated by the Church authorities and
convicted of teaching that the earth went around the sun as a matter of fact and not, as he had promised previously,
as a mere mathematical hypothesis. He was placed under permanent house arrest, and forbidden to write about or
teach his theories. Immediately after being forced to recant his claim that the earth revolved around the sun, the old
man is said to have muttered defiantly “and yet it does move.”
The story is dramatic, but there are some omissions in the commonly taught heroic version. There was a rumor
that the Simplicio character represented the Pope. Also, some of the ideas Galileo advocated had controversial religious
overtones. He believed in the existence of atoms, and atomism was thought by some people to contradict the Church’s
doctrine of transubstantiation, which said that in the Catholic mass, the blessing of the bread and wine literally
transformed them into the flesh and blood of Christ. His support for a cosmology in which the earth circled the sun
was also disreputable because one of its supporters, Giordano Bruno, had also proposed a bizarre synthesis of Christianity
with the ancient Egyptian religion.
3.1The Motion of Falling Objects
The motion of falling objects is the simplest and most common ex-
ample of motion with changing velocity. The early pioneers of physics had a
correct intuition that the way things drop was a message directly from
Nature herself about how the universe worked. Other examples seem less
likely to have deep significance. A walking person who speeds up is making
a conscious choice. If one stretch of a river flows more rapidly than another,
it may be only because the channel is narrower there, which is just an
accident of the local geography. But there is something impressively consis-
tent, universal, and inexorable about the way things fall.
Stand up now and simultaneously drop a coin and a bit of paper side by
side. The paper takes much longer to hit the ground. That’s why Aristotle
wrote that heavy objects fell more rapidly. Europeans believed him for two
thousand years.
Now repeat the experiment, but make it into a race between the coin
and your shoe. My own shoe is about 50 times heavier than the nickel I had
handy, but it looks to me like they hit the ground at exactly the same
moment. So much for Aristotle! Galileo, who had a flair for the theatrical,
did the experiment by dropping a bullet and a heavy cannonball from a tall
tower. Aristotle’s observations had been incomplete, his interpretation a vast
It is inconceivable that Galileo was the first person to observe a discrep-
ancy with Aristotle’s predictions. Galileo was the one who changed the
course of history because he was able to assemble the observations into a
coherent pattern, and also because he carried out systematic quantitative
(numerical) measurements rather than just describing things qualitatively.
Why is it that some objects, like the coin and the shoe, have similar
motion, but others, like a feather or a bit of paper, are different. Galileo
3Acceleration and
Free Fall
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