The Science Book

SCIENTIFIC REVOLUTION 65

taught that moving objects only
kept moving as long as they were
being pushed, and that heavy
objects fell faster than lighter ones.
Aristotle explained that heavy
objects fell to Earth because they
were moving toward their natural
place. He also said that celestial
bodies, being perfect, must all
move in circles at constant speeds.
Galileo Galilei came up with
a different set of ideas, arrived at
through experiment. He observed
balls running down ramps and
demonstrated that objects all fall
at the same rate if air resistance
is minimal. He also concluded that
moving objects continue to move
unless a force, such as friction,
acts to slow them down. Galileo’s
Principle of Inertia was to
become part of Newton’s First
Law of Motion. Since friction and
air resistance act on all moving
objects that we encounter in daily
life, the concept of friction is not
immediately obvious. It was only
by careful experimentation that
Galileo could show that the force
keeping something moving at a
steady speed was only needed to
counteract friction.

Laws of motion
Newton experimented in many
areas of interest, but no records of
his experiments on motion survive.
His three laws, however, have been
verified in many experiments,
holding true for speeds well below
the speed of light. Newton stated
his first law as: “Every body
perseveres in its state of rest, or
of uniform motion in a right line,
unless it is compelled to change that
state by forces impressed thereon.”
In other words, a stationary
object will only start to move if

a force acts on it, and a moving

object continues to move with

constant velocity unless a force

acts on it. Here, velocity means

both the direction of a moving

object and its speed. So an object

will only change its speed or

change direction if a force acts on

it. The force that is important is the

net force. A moving car has many

forces on it, including friction and

air resistance, and also the engine

driving the wheels. If the forces

See also: Nicolaus Copernicus 34–39 ■ Johannes Kepler 40–41 ■ Galileo Galilei 42–43 ■ Christiaan Huygens 50–51 ■
William Herschel 86–87 ■ Albert Einstein 214–21

pushing the car forward balance

the forces trying to slow it down,

there is no net force and the car

will maintain a constant velocity.

Newton’s Second Law states

that the acceleration (a change

of velocity) of a body depends on

the size of the force acting on it,

and is often written down as

F = ma, where F is force, m is mass,

and a is acceleration. This shows

that the greater the force on a body,

the greater the acceleration. ❯❯

Rocket engines

are an example of

Newton’s Third Law

in action. The rocket

produces a jet that is

forced downward.

The jet exerts an equal

and opposite force that

pushes the rocket up.

Rocket

pushed

up

Exhaust flow

pushed down