Thinking and Deciding

18 WHAT IS THINKING?
new concepts. For instance, whereas some young children believe that the sun is
alive, sleeps at night, and could stop shining if it wanted to, older children learn that
the sun is a star like the others that shine at night. Second, the relationships among
the child’s concepts change. The earth is no longer physically at the center of the
universe; the light from the moon is understood as related to the positions of the
earth and sun. Finally, the new system explains different phenomena. It explains
the relationship between sun, moon, and stars, but not the relationship between sun,
clouds, and wind (which might have been understood as being interrelated in the old
system). This last point is of interest because it suggests that something is lost by
adopting the new system — not just the innocence of childhood, but also a way of
understanding certain things. The appearance of the earth as being flat and of the
sun as being much smaller than the earth now become mysteries. Eventually, these
things too will be explained, of course. In sum, as children adopt adult astronomy
there are changes of belief that are as radical as those that have occurred in history,
from the system of ancient astronomy to the system of Copernicus.
Just as children have naive theories of astronomy, children and adults seem to
have naive theories in other subject areas, such as physics, which must be replaced,
sometimes with great difficulty, in order for a person to learn a modern scientific
theory. Often these naive theories correspond to systems proposed by early scientists
such as Aristotle. (We shall see in the last two parts of this book that there may also
be naive theories of judgment and decision making, theories held by most adults.)
Clement (1983), for example, found that students who had taken a physics course
held a theory (sometimes even after they had finished the course) about physical
forces that was similar to one held by Galileo in his early work but that later both he
and Newton questioned. The students believed that a body in motion always requires
a force to keep it in motion. In contrast, we now find it more useful to suppose
that a body keeps going unless it is stopped or slowed by some force. Of course,
wagons and cars do require force to keep them going, but that is because they would
otherwise be slowed down by friction.
Clement asked his subjects what forces were acting on a coin thrown up into the
air, during the time it was rising but after it had left the thrower’s hand. Most students
(even after taking the course) said that there was a force directed upward while the
coin was rising, in addition to the force of gravity pulling the coin down. This view
fits in with the “motion implies force” theory that the students held. Physicists find
it more useful to suppose that there is only one force, the force of gravity, once the
coin is released.
Clement also asked the students about the forces acting on the bob of a pendulum
while the pendulum is swinging. Many students said that there were three forces: (1)
the force of gravity, pulling the bob straight down; (2) the force of the string, pulling
the bob directly toward the point where the string was attached; and (3) a force acting
in the direction in which the bob was moving, as shown in the following diagram. A
modern physicist would say that the third force is unnecessary to explain the motion
of the bob.