42
A FALLING BODY
ACCELERATES
UNIFORMLY
GALILEO GALILEI (1564–1642)
F
or 2,000 years, few people
challenged Aristotle’s
assertion that an external
force keeps things moving and that
heavy objects fall faster than lighter
ones. Only in the 17th century
did the Italian astronomer and
mathematician Galileo Galilei
insist that the ideas had to be
tested. He devised experiments
to test how and why objects move
and stop moving, and was the first
to figure out the principle of
inertia—that objects resist a
change in motion and need a force
to start moving, speed up, or slow
down. By timing objects falling,
Galileo showed that the rate of fall
is the same for all objects, and
came to realize the part played by
friction in slowing them down.IN CONTEXT
BRANCH
PhysicsBEFORE
4th century BCE Aristotle
develops ideas about forces
and motion, but does not test
them experimentally.1020 Persian scholar Ibn Sina
(Avicenna) writes that moving
objects have innate “impetus,”
slowed only by external factors
such as air resistance.1586 Flemish engineer Simon
Stevin drops two lead balls of
unequal weight from a church
tower in Delft to show that
they fall at the same speed.AFTER
1687 Isaac Newton’s Principia
formulates his laws of motion.1971 US astronaut Dave Scott
demonstrates Galileo’s ideas
about falling bodies by
showing that a hammer and a
feather fall at the same rate on
the Moon, which has almost
no atmosphere to cause drag.With the equipment available
during the 1630s, Galileo could
not directly measure the speed or
acceleration of freely falling objects.
By rolling balls down one ramp and
up another, he showed that the
speed of a ball at the bottom of
the ramp depended on its starting
height, not on the steepness of the
ramp, and that a ball would always
roll up to the same height it had
started from, no matter how steep
or shallow the inclines were.
Galileo carried out his remaining
experiments with a ramp 16 ft (5 m)
long, lined with a smooth material to
reduce friction. For timing, he used a
large container of water with a small
pipe in the bottom. He collected the
water during the interval he was
measuring, and weighed the waterGalileo demonstrated that the speed a ball
reaches at the bottom of a ramp depends only on
its starting height, not the steepness of the ramp.
Here, balls dropped from points A and B will
reach the bottom of the ramp at the same speed.A B