80 PART 1^ |^ EXPLORING THE SKY
velocity and requires the presence of a force. Aristotle said that
objects move because they have a tendency to move. Newton said
that objects move due to a specifi c cause, a force.
Newton’s third law of motion specifi es that for every action there
is an equal and opposite reaction. In other words, forces must occur in
pairs directed in opposite directions. For example, if you stand on a
skateboard and jump forward, the skateboard will shoot away back-
ward. As you jump, your feet must exert a force against the skateboard,
which accelerates it toward the rear. But forces must occur in pairs, so
the skateboard must exert an equal but opposite force on your feet,
and that is what accelerates your body forward (Figure 5-5c).very little result. If it is full of infl ated balloons, however, it will
move easily in response to a gentle push. Finally, the second law
says that the resulting acceleration is in the direction of the force.
Th is is also what you would expect. If you push on a cart that is not
moving, you expect it to begin moving in the direction you push.
Th e second law of motion is important because it establishes
a precise relationship between cause and eff ect (How Do We
Know? 5-1). Objects do not just move. Th ey accelerate due to
the action of a force. Moving objects do not just stop. Th ey
decelerate due to a force. And moving objects don’t just change
direction for no reason. Any change in direction is a change inReactionActionFF=maa
ma b c■ Figure 5-5
Newton’s three laws of
motion.The So-Called Scientifi c Method
Why is the principle of cause and effect
so important to scientists? One of the most
often used and least often stated principles of
science is cause and effect. Modern scientists
all believe that events have causes, but ancient
philosophers such as Aristotle argued that
objects moved because of tendencies. They said
that earth and water, and objects made mostly
of earth and water, had a natural tendency to
move toward the center of the universe. This
natural motion had no cause but was inherent
in the nature of the objects. Newton’s second
law of motion (F = ma) was the fi rst clear
statement of the principle of cause and effect.
If an object (of mass m) changes its motion (a
in the equation), then it must be acted on by a
force (F in the equation). Any effect (a) must
be the result of a cause (F).
The principle of cause and effect goes far
beyond motion. It gives scientists confi dence
that every effect has a cause. The struggleagainst disease is an example. Cholera is a
horrible disease that can kill its victims in
hours. Long ago it was probably blamed on
bad magic or the will of the gods, and only
two centuries ago it was blamed on “bad air.”
When an epidemic of cholera struck England in
1854, Dr. John Snow carefully mapped cases
in London showing that the victims had drunk
water from a small number of wells contami-
nated by sewage. In 1876, the German Dr.
Robert Koch traced cholera to an even more
specifi c cause when he identifi ed the micro-
scopic bacillus that causes the disease. Step
by step, scientists tracked down the cause of
cholera.
If the universe did not depend on cause
and effect, then you could never expect to
understand how nature works. Newton’s second
law of motion was arguably the fi rst clear
statement that the behavior of the universe
depends rationally on causes.Cause and effect: Why did this star explode in
1992? There must have been a cause. (ESA/STScI
and NASA)