The Solar System

78 PART 1^ |^ EXPLORING THE SKY

Th e book is a brilliant achievement for a number of reasons.

To understand motion, Galileo had to abandon the authority of

the ancients, devise his own experiments, and draw his own con-

clusions. In a sense, this was the fi rst example of experimental

science. But Galileo also had to generalize his experiments to

discover how nature worked. Th ough his apparatus was fi nite

and his results skewed by friction, he was able to imagine an

infi nite, frictionless plane on which a body moves at constant

velocity. In his workshop, the law of inertia was obscure, but in

his imagination it was clear and precise.

Newton and the Laws of Motion

From the work of Galileo, Kepler, and other early scientists, Isaac

Newton was able to deduce three laws of motion (■ Table 5-1)

that describe any moving object, from an automobile driving

along a highway to galaxies colliding with each other. Th ose laws

led Newton to an understanding of gravity.

Galileo published his work on motion in 1638, two years
after he had become entirely blind and only four years before his
death. Th e book was called Mathematical Discourses and
Demonstrations Concerning Two New Sciences, Relating to Mechanics
and to Local Motion. It is known today as Two New Sciences.

1 s

9.8 m/s

2 s

19.6 m/s

3 s

29.4 m/s

Air resistance would have

slowed the wooden ball more and

ruined Galileo’s demonstration.

On the airless moon, there is no

air resistance to slow the feather.

a

b

■ Figure 5-3

Galileo found that a falling object is accelerated
downward. Each second, its velocity increases by
9.8 m/s (32 ft/s).

■ Figure 5-4

(a) According to tradition, Galileo demonstrated that the acceleration of a falling body is independent

of its weight by dropping balls of iron and wood from the Leaning Tower of Pisa. In fact, air resistance

would have confused the result. (b) In a historic television broadcast from the moon on August 2, 1971,

David Scott dropped a hammer and a feather at the same instant. They fell with the same acceleration

and hit the surface together. (NASA)