Between 1665 and 1667, Isaac Newton made revolutionary discoveries regarding motion, gravity, light, and mathematics. While avoiding the plague in London, Newton developed his theory of inertia, which states that an object at rest stays at rest and an object in motion stays in motion unless acted upon by an external force. He determined that a constant force is not needed to keep an object in motion, overturning prior scientific theories. Newton's theory of inertia formed the basis for his first law of motion.

1. Sir lseieic Newton's
First leiw
Reading Focus What is inertia?
Isaac Newton was born in England in
1642,just a few months after the death
of Galileo. As a young man, Newton
spent most of his time working on his
inventions. These included kites, a sundial,
and a water clock.
Newton graduated from Cambridge
University in 1665, but he made no par-
ticular impression on his teachers. As a
student, he did not seem destined to
become one of the great scientific geniuses
of all time.
Newton's Thoughts
About Motion
Shortly after his graduation, Newton's
life suddenly changed. A terrible epidemic
known as the plague swept through
Cambridge in 1665. To avoid the disease,
Newton moved from Cambridge to his
mother's farm near London. During the
next 18 months, he made some of the
most important discoveries in the history
of mathematics and science.
Among Newton's discoveries were his
hypotheses about the nature of motion.
Until this time, scientists thought that a
constant force was needed to keep an
object in motion. It seemed obvious to
them that if a force stopped acting on an
object, the object slowed down and even-
tually stopped.
Newton thought differently. A story is
often told that while sitting under atree,
F58
A Between 1665 and 1667, Isaac Newton
revolutionized science with discoveries
regarding gravity, motion, light, and
mathematics.
he was struck on the head by a falling
apple. While we don't know that this
actually happened, we do know that late
in his life Newton himself told the story
to a friend. He explained that he had
wondered what caused the apple's
motion. Before long, Newton was look-
ing beyond the problem of a single apple
dropping from a tree. He extended his
inquiries to include the causes of the
movement of Earth, the Moon, the
planets, and the stars.

2. The Concept of lnertia
Over time, Newton worked out a num-
ber of hypotheses explaining the nature
of motion. The basis of all these hypoth-
eses is now known as Newton's first law
of motion. According to this law, objects
at rest tend to remain at rest. Objects in
motion tend to stay in motion, traveling
at a constant speed and in the same
direction. The tendency of an object to
-*-'
remain at rest or to remain in motion is
called inertia.
We expect a rock to remain sitting at
the top of a hill. Similarly, a ball rolling
across a table top might be expected to
continue rolling in the same direction; no
force is needed to keep it in motion.
What we do have to explain are
changes in the conditions that influence
an object's motion. For example, sup-
pose that the rock starts rolling down the
hill. The problem is to explain why the
rock changed its state. What change in
conditions made the rock start rolling
down the hill?
Or suppose that a rolling ball speeds
up, slows down, or changes direction.
You'll discover Newton's explanation for
those changes in the ball's condition in
the next investigation.
Examples of lnertia
You can find many examples of inertia
in everyday life. In the activity on pages
F56 and F57, for example, inertia causes
the crash dummy to change position
when the truck stops suddenly. Similarly,
suppose you are standing in the aisle of a
bus that is traveling at a constant speed
of 40 km/h (25 mph). What would
happen if the driver suddenly slammed
on the brakes? Chances are you'd fall
forward as the bus came to a stop. This
would happen because your body has
A Unless acted on by some force, this rock
will forever balance on top of the other
rock. What measurable forces could
affect the balance of the rock?
inertia while the bus is traveling forward.
Your body is moving in the same
direction and at the same speed as the
bus. When the driver hits the brakes, the
bus comes to a stop, but inertia keeps
your body moving forward.
A similar explanation applies to sudden
starts by the bus. You stand quietly in the
aisle while the bus driver waits for the last
passenger to board. At some point, the
driver suddenly steps on the accelerator.
The bus lurches forward. What happens to
you and the other passengers? How does
inertia explain the changes that occur? I
F59