Thinking and Deciding

KNOWLEDGE, THINKING, AND UNDERSTANDING 19
In another series of studies, McCloskey (1983) asked undergraduates, some of
whom had studied physics, to trace the path of a metal ball shot out of a curved tube
at high speed, as shown in the following illustration. The tube is lying flat on top
of a surface; therefore the effect of gravity can be ignored. Many of the subjects,
including some who had studied physics, said that the path of the ball would be
curved. In fact, it would be straight. McCloskey argues (on the basis of interviews
with subjects) that these students held a theory in which the ball acquires some sort
of “impetus,” a concept something like the mature concept of momentum, except that
the impetus can include the curvature of the path. A similar theory was apparently
held during medieval times.
Roncato and Rumiati (1986) showed subjects a drawing like the following, 3
which shows two bars, each supported by a cable from the ceiling, attached to a
pivot through the bar, around which the bar could turn freely. The thicker lines under
each bar are supports. What happens when the supports are carefully removed?
(Think about it.) Most subjects thought that the first bar would become horizontal
and the second bar would remain tilted (although perhaps at a different angle). They
seemed to think that the angle of the bar would indicate the discrepancy between the
weight on the two sides, as a balance scale would do. In fact, the first bar would
3 Reproduced with the authors’ permission.